Colors of Twilight vs. Sunset/Sunrise

[Peter87]

All I can say is that purple light triggers a response from both the blue-sensitive and the red-sensitive rods in our retinas. Consider this color, ███. The (rgb) values of this particular color is, r = 123, g = 0, and b = 222. So purple, at least as far as our eyes are concerned, is indeed a mixture of red and blue.

I can see that the mixture of red and blue light in the sky under certain circumstances will make parts of even in the cloudless dawn or dusk sky look purple (because the atmosphere itself will reflect light and provide some mixing of the colors). I know I have seen clouds reflecting the dawn or dusk light look purple (but mixed with other colors as well).

I have seen the sky look like this. Credit: EIJI OGURA/EYEEM/GETTY IMAGES

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I have seen the sky look like this. Credit: I don't know.

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Ann

Ann, that’s very interesting to see the purple color with rgb values of r = 123, g = 0, and b = 222. The b = 222 is a high value -- approaching the maximum 255.

I am wondering how to describe the blue in the upper atmosphere behind the purple clouds in the first photo. It looks as if it is during blue hour. Do you think blue rgb values of blue hour would be lower than 222?

[Ann]

All I can say is that purple light triggers a response from both the blue-sensitive and the red-sensitive rods in our retinas. Consider this color, ███. The (rgb) values of this particular color is, r = 123, g = 0, and b = 222. So purple, at least as far as our eyes are concerned, is indeed a mixture of red and blue.

I can see that the mixture of red and blue light in the sky under certain circumstances will make parts of even in the cloudless dawn or dusk sky look purple (because the atmosphere itself will reflect light and provide some mixing of the colors). I know I have seen clouds reflecting the dawn or dusk light look purple (but mixed with other colors as well).

I have seen the sky look like this. Credit: EIJI OGURA/EYEEM/GETTY IMAGES

---

I have seen the sky look like this. Credit: I don't know.

---

Ann

Ann, that’s very interesting to see the purple color with rgb values of r = 123, g = 0, and b = 222. The b = 222 is a high value -- approaching the maximum 255.

I am wondering how to describe the blue in the upper atmosphere behind the purple clouds in the first photo. It looks as if it is during blue hour. Do you think blue rgb values of blue hour would be lower than 222?

I'm not sure that the picture taken by Eiji Ogura was taken during the blue hour. It might have been. I think it is more likely that the other picture was. The clouds reflected the colors of the reddened Sun after it had sunk below the horizon.

Wikipedia shows us this cloudless picture of the blue hour:

Praia da Ursa, Cabo da Roca, Portugal during the blue hour. Credit: Rnbc

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My guess is that the b value of the sky during the blue hour would be 255.


I found an interesting chart showing the hues, hex codes and rgb values for several shades of purple. There is always more b than r in all these shades of purple, and the b value is at or close to 255 in all the shades of purple except the darkest ones.

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Ann

[Chris Peterson]

Thanks for the suggestion, Chris, I didn't think of that!

That's a lovely goat, by the way. What's her name? She looks very feminine to me!

Ann

Gretel. She'll be kidding in a couple of weeks.

Thanks for teaching me a new word, Chris! I only knew one meaning of the word "kidding".

I'm sure Gretel's kids will be adorable!

Ann

[Ann]

Gretel. She'll be kidding in a couple of weeks.

Thanks for teaching me a new word, Chris! I only knew one meaning of the word "kidding".

I'm sure Gretel's kids will be adorable!

Ann

Click to view full size image

Oh no no no, I can't stand the cuteness of this baby! Is the little one a boy or a girl?

Couldn't find a good goat emoji, so I guess a unicorn is the closest thing!

Ann

[Ann]

Very true... but possibly excepting flowers! The color of the petals of pink/red/blue/purple flowers is created by a class of molecules called anthocyanins, and the actual color can vary across that range, determined primarily by pH. So really, there is no "natural" or "real" color for these flowers, as subtle genetic and environmental variation can create a wide range within a single species. The flowers you imaged might appear bluer because of the nature of your phone camera, but they could be genuinely bluer, as well.

One lab I do in my science classes is to take purple cabbage and boil it for a few minutes, then collect the purple water. This can be titrated with an acid like vinegar or a base like baking soda to instantly change the color over a wide range. Great fun for the kids. Looks like magic.

Well, Chris, I did the experiment. I got a very small piece of red cabbage from my friend, boiled it and then poured the purple water into two glasses.

Click to view full size image

I then put baking soda into the glass at left and vinegar into the glass at right. The liquid in the glass at right instantly changed color to a bright red hue. But the liquid in the glass at left, where I had put baking soda, remained very dark, almost black-looking.

Click to view full size image

So I decided that I had to dilute the cabbage water into which I had put baking soda. I poured water into a smaller glass and added just a bit of the dark-colored cabbage water that contained baking soda. Now this liquid became clearly blue.

Click to view full size image

You can see the poor cabbage leaves on the small plate in the background.

All in all, Chris, I guess you can say that the experiment worked.

Ann

Try it with a little (chlorine) bleach.

Well, Chris, I finally got around to trying this experiment again (even having bought a humongous bottle of bleach just for this purpose - I couldn't get anything smaller).

The failure was spectacular!!!

That looks okay...

That doesn't!!!

Frankly, Chris, I'm done with the cabbage experiment now!
And, hey, if anyone is interested in buying a barrel of bleach...

Ann

[Peter87]

I wonder if this Wikipedia article on Sky brightness makes sense -- it seems to that there is both direct and indirect sunlight in the sky at twilight --

"Indirectly scattered sunlight comes from ... the atmosphere itself .... the Sun has just set but still illuminates the upper atmosphere directly." https://en.wikipedia.org/wiki/Sky_brigh ... atmosphere.

I don't know if that makes sense in one way of distinguishing the colors of twilight and sunset/sunrise -- at sunset/sunrise, all or most of the visible sunlight in the sky is direct, at twilight, both direct and indirect?

[Chris Peterson]

I wonder if this Wikipedia article on Sky brightness makes sense -- it seems to that there is both direct and indirect sunlight in the sky at twilight --

"Indirectly scattered sunlight comes from ... the atmosphere itself .... the Sun has just set but still illuminates the upper atmosphere directly." https://en.wikipedia.org/wiki/Sky_brigh ... atmosphere.

I don't know if that makes sense in one way of distinguishing the colors of twilight and sunset/sunrise -- at sunset/sunrise, all or most of the visible sunlight in the sky is direct, at twilight, both direct and indirect?

The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.

[Peter87]

The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.

Regarding the lower blue part, which is the Earth's shadow, I wonder if the blue color is not a function of Rayleigh scattering but instead Chappuis absorption ("when solor rays graze the stratosphere the Chappuis band of ozone extinguishes the longer wavelengths, leaving only blue" https://adsabs.harvard.edu/full/2005JBAA..115..247L)?

[Chris Peterson]

The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.

Regarding the lower blue part, which is the Earth's shadow, I wonder if the blue color is not a function of Rayleigh scattering but instead Chappuis absorption ("when solor rays graze the stratosphere the Chappuis band of ozone extinguishes the longer wavelengths, leaving only blue" https://adsabs.harvard.edu/full/2005JBAA..115..247L)?

It doesn't look ozone blue to me. I'd say it's dominated by Rayleigh scattered sunlight, which itself is scattered off the illuminated part of the sky.

[Peter87]

It doesn't look ozone blue to me. I'd say it's dominated by Rayleigh scattered sunlight, which itself is scattered off the illuminated part of the sky.

I see, perhaps because the Belt of Venus is visible shortly before sunrise or after sunset, during civil twilight, and ozone blue occurs closer to nautical twilight and blue hour?

[Peter87]

The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.

When I think of Descartes on clear and distinct ideas, where “distinct” perceptions are "sharply separated from all other perceptions" –- then I struggle to distinguish between the upper pink part of the Belt of Venus, which is directly lit by the Sun, from the lower blue part, which is only scattering light from other parts of the sky.

The lower blue part, which is only scattering light from other parts of the sky, means that the source of this light is indirect, it hits the upper atmosphere, bounces off, and then cascades downward, so to speak?





(My apologies, I regret having to pose this question that has such a self-evident answer. It reveals my poor background in astronomy.)

[Chris Peterson]

The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.

When I think of Descartes on clear and distinct ideas, where “distinct” perceptions are "sharply separated from all other perceptions" –- then I struggle to distinguish between the upper pink part of the Belt of Venus, which is directly lit by the Sun, from the lower blue part, which is only scattering light from other parts of the sky.

The lower blue part, which is only scattering light from other parts of the sky, means that the source of this light is indirect, it hits the upper atmosphere, bounces off, and then cascades downward, so to speak?





(My apologies, I regret having to pose this question that has such a self-evident answer. It reveals my poor background in astronomy.)

The lower blue part of the Belt is just the twilight sky rising (or setting). In the evening I sometimes call it "nightrise". So yeah, it is only lit indirectly, by light that has scattered from air still lit by the Sun directly.

[Peter87]

The lower blue part of the Belt is just the twilight sky rising (or setting). In the evening I sometimes call it "nightrise". So yeah, it is only lit indirectly, by light that has scattered from air still lit by the Sun directly.

So every moment of twilight involves some combination of direct and indirect sunlight, it seems. At the latest/earliest moments of astronomical twilight, there is a very minimal amount of direct sunlight arrivng at a very steep angle -- and the direct sunlight hits the upper atmosphere, into the ozone layer, takes on the characteristics of ozone blue, and "bounces down" as indirect light?

("bounces down" would be a metaphor I would be able to grasp)

[Chris Peterson]

The lower blue part of the Belt is just the twilight sky rising (or setting). In the evening I sometimes call it "nightrise". So yeah, it is only lit indirectly, by light that has scattered from air still lit by the Sun directly.

So every moment of twilight involves some combination of direct and indirect sunlight, it seems. At the latest/earliest moments of astronomical twilight, there is a very minimal amount of direct sunlight arrivng at a very steep angle -- and the direct sunlight hits the upper atmosphere, into the ozone layer, takes on the characteristics of ozone blue, and "bounces down" as indirect light?

("bounces down" would be a metaphor I would be able to grasp)

I'm skeptical that we ever see ozone blue. Twilight is when the sky is lit by indirect light coming from parts of the sky in the direction of the Sun, mostly or completely over the horizon.

[Peter87]

The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.

I can't seem to figure out, why is the Belt of Venus pink as opposed to, say, red. Red is a predictable in sunsets, of course, one finds many explanations about it, but pink, I have not been able to find any explanations for its occurrence...

[Chris Peterson]

The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.

I can't seem to figure out, why is the Belt of Venus pink as opposed to, say, red. Red is a predictable in sunsets, of course, one finds many explanations about it, but pink, I have not been able to find any explanations for its occurrence...

It's being illuminated by the sunset... which is red/orange/pink. That is, the Sun that is lighting its upper part has passed through a very long atmospheric path and had most of its short wavelength light scattered away.

[Peter87]

It's being illuminated by the sunset... which is red/orange/pink. That is, the Sun that is lighting its upper part has passed through a very long atmospheric path and had most of its short wavelength light scattered away.

Thank you so much for all of your insights... I wonder if this could be specified a bit more -- one might think, it is remarkable that the Belt of Venus is predominantly predictably pink while sunsets are not predominantly predictably pink.

Greater specificity might be warranted here: The sunset (sun above horizon) and twilight (sun below horizon) are both red (but different reds) for the same reason (thus far explained) that the Belt of Venus is pink: in both cases, sunlight has passed through a very long atmospheric path and had most of its short wavelength light scattered away(?)

One might think, the sunset/twilight is not predominantly predictably pink while the Belt is predominantly predictably pink.

Hypothesis: because the Belt of Venus results from backscattering, as a result of backscattering the red waves -- "weaken"? / "lose their strength"? / "dissipate"? -- and thus are pink, a lightened form of red, where pink = red + white... (?)

[Peter87]

Apparently, Alpenglow and the Belt of Venus are the essentially the same phenomenon, "where predominantly red light is scattered by particulates, forming a pink glow" https://www.colorado.edu/asmagazine/202 ... -milky-way

The Belt of Venus, as with alpenglow, results from "predominantly red light is scattered by particulates, forming a pink glow" https://en.wikipedia.org/wiki/Alpenglow

But it is a mystery, perhaps unsolved in physics/astonomy -- why pink and not red?

It is a paradox, that it remains unexplained -- why exactly pink and not red? -- because Belt of Veus/Aphrodite is a matter of how physics overlaps with both aesthetics and mythology, physics/astronomy coinciding with what is beautiful and what is one of the West's deepest, most ancient foundations -- myths of Greek gods -- and it is here where physics/astronomy remains silent -- and so the connection between science and the humanities seems severed here...

[Chris Peterson]

Apparently, Alpenglow and the Belt of Venus are the essentially the same phenomenon, "where predominantly red light is scattered by particulates, forming a pink glow" https://www.colorado.edu/asmagazine/202 ... -milky-way

The Belt of Venus, as with alpenglow, results from "predominantly red light is scattered by particulates, forming a pink glow" https://en.wikipedia.org/wiki/Alpenglow

But it is a mystery, perhaps unsolved in physics/astonomy -- why pink and not red?

It is a paradox, that it remains unexplained -- why exactly pink and not red? -- because Belt of Veus/Aphrodite is a matter of how physics overlaps with both aesthetics and mythology, physics/astronomy coinciding with what is beautiful and what is one of the West's deepest, most ancient foundations -- myths of Greek gods -- and it is here where physics/astronomy remains silent -- and so the connection between science and the humanities seems severed here...

Pink and red are the same hue, just varying by saturation. Saturation in this case is determined by how much white light (or at least, not red light) is mixed in.

So first... the light source itself is not red, it is a broad spectrum that is more intense at longer wavelengths, but is far from a stop light! And then, we have a lot of light from the rest of the sky scattered in, and that scatter favors shorter wavelengths. So the color is diluted (desaturated). And the nature of the scatter (as you alluded to above) has to be considered, as well. We have both Rayleigh scattering from molecules in the atmosphere, with forward and back scatter equal, and we have Mie scattering from particles and aerosols, which has a complex scatter pattern favoring forward scatter.

[Peter87]

Pink and red are the same hue, just varying by saturation. Saturation in this case is determined by how much white light (or at least, not red light) is mixed in.

So first... the light source itself is not red, it is a broad spectrum that is more intense at longer wavelengths, but is far from a stop light! And then, we have a lot of light from the rest of the sky scattered in, and that scatter favors shorter wavelengths. So the color is diluted (desaturated). And the nature of the scatter (as you alluded to above) has to be considered, as well. We have both Rayleigh scattering from molecules in the atmosphere, with forward and back scatter equal, and we have Mie scattering from particles and aerosols, which has a complex scatter pattern favoring forward scatter.

Chris, thanks for the very clearly specified explanation, greatly appreciated. Yes, I see, the light source itself is not red, it is a broad spectrum that is more intense at longer wavelengths, but not as intense as a stop light -- I like the comparison! The color is diluted (desaturated), thus a predominantly pink Belt of Venus/antitwilight arch.

Because of nature of the scatter, which involves a backscattering, this is complicit in creating the desaturation; backscattering -----> desaturation?

The color is more "intense" -- "intense" is a technical term in physics/astronomy? Or somewhat of a metaphor, as people also say, intense is blazing, bright, brilliant, vivid, vibrant... But intense as "glowing" and "radiant" seem more scientifically precise?

Does it sound reasonable to say that the Belt of Venus is "glowing" somewhat, especially in its early stages?

[Peter87]

Pink and red are the same hue, just varying by saturation. Saturation in this case is determined by how much white light (or at least, not red light) is mixed in.

So first... the light source itself is not red, it is a broad spectrum that is more intense at longer wavelengths, but is far from a stop light! And then, we have a lot of light from the rest of the sky scattered in, and that scatter favors shorter wavelengths. So the color is diluted (desaturated). And the nature of the scatter (as you alluded to above) has to be considered, as well. We have both Rayleigh scattering from molecules in the atmosphere, with forward and back scatter equal, and we have Mie scattering from particles and aerosols, which has a complex scatter pattern favoring forward scatter.

Chris, thanks for the very clearly specified explanation, greatly appreciated. Yes, I see, the light source itself is not red, it is a broad spectrum that is more intense at longer wavelengths, but not as intense as a stop light -- I like the comparison! The color is diluted (desaturated), thus a predominantly pink Belt of Venus/antitwilight arch.

Because of nature of the scatter, which involves a backscattering, this is complicit in creating the desaturation; backscattering -----> desaturation?

The color is more "intense" -- "intense" is a technical term in physics/astronomy? Or somewhat of a metaphor, as people also say, intense is blazing, bright, brilliant, vivid, vibrant... But intense as "glowing" and "radiant" seem more scientifically precise?

Does it sound reasonable to say that the Belt of Venus is "glowing" somewhat, especially in its early stages?

[Chris Peterson]

Pink and red are the same hue, just varying by saturation. Saturation in this case is determined by how much white light (or at least, not red light) is mixed in.

So first... the light source itself is not red, it is a broad spectrum that is more intense at longer wavelengths, but is far from a stop light! And then, we have a lot of light from the rest of the sky scattered in, and that scatter favors shorter wavelengths. So the color is diluted (desaturated). And the nature of the scatter (as you alluded to above) has to be considered, as well. We have both Rayleigh scattering from molecules in the atmosphere, with forward and back scatter equal, and we have Mie scattering from particles and aerosols, which has a complex scatter pattern favoring forward scatter.

Chris, thanks for the very clearly specified explanation, greatly appreciated. Yes, I see, the light source itself is not red, it is a broad spectrum that is more intense at longer wavelengths, but not as intense as a stop light -- I like the comparison! The color is diluted (desaturated), thus a predominantly pink Belt of Venus/antitwilight arch.

Because of nature of the scatter, which involves a backscattering, this is complicit in creating the desaturation; backscattering -----> desaturation?

The color is more "intense" -- "intense" is a technical term in physics/astronomy? Or somewhat of a metaphor, as people also say, intense is blazing, bright, brilliant, vivid, vibrant... But intense as "glowing" and "radiant" seem more scientifically precise?

Does it sound reasonable to say that the Belt of Venus is "glowing" somewhat, especially in its early stages?

Well, most of those terms (at least in this context) are pretty subjective, but I think most people understand what they mean (again, in this context) and are reasonable for their descriptive intent.

[Peter87]

The Belt of Venus illustrates this. The upper pink part is directly lit by the Sun. The lower blue part is only scattering light from other parts of the sky.

Yes, I agree, most of those terms are subjective. I was thinking about how the upper pink part of the Belt of Venus is directly lit by the Sun and the lower blue part is only scattering light from other parts of the sky --

-- does it seem correct to you to say that the desaturated but relatively intense pink color of the upper pink part of the Belt of Venus is intense precisely because it is directly lit by the Sun, while the lower blue part is not intense at all because it is only scattering light from other parts of the sky and not directly lit by the Sun?

~Thanks

[Peter87]

All I can say is that purple light triggers a response from both the blue-sensitive and the red-sensitive rods in our retinas. Consider this color, ███. The (rgb) values of this particular color is, r = 123, g = 0, and b = 222. So purple, at least as far as our eyes are concerned, is indeed a mixture of red and blue.

I can see that the mixture of red and blue light in the sky under certain circumstances will make parts of even in the cloudless dawn or dusk sky look purple (because the atmosphere itself will reflect light and provide some mixing of the colors). I know I have seen clouds reflecting the dawn or dusk light look purple (but mixed with other colors as well).

I have seen the sky look like this. Credit: EIJI OGURA/EYEEM/GETTY IMAGES

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I have seen the sky look like this. Credit: I don't know.

---

Ann

Ann, that’s very interesting to see the purple color with rgb values of r = 123, g = 0, and b = 222. The b = 222 is a high value -- approaching the maximum 255.

I am wondering how to describe the blue in the upper atmosphere behind the purple clouds in the first photo. It looks as if it is during blue hour. Do you think blue rgb values of blue hour would be lower than 222?

I'm not sure that the picture taken by Eiji Ogura was taken during the blue hour. It might have been. I think it is more likely that the other picture was. The clouds reflected the colors of the reddened Sun after it had sunk below the horizon.

Wikipedia shows us this cloudless picture of the blue hour:

Praia da Ursa, Cabo da Roca, Portugal during the blue hour. Credit: Rnbc

---

My guess is that the b value of the sky during the blue hour would be 255.


I found an interesting chart showing the hues, hex codes and rgb values for several shades of purple. There is always more b than r in all these shades of purple, and the b value is at or close to 255 in all the shades of purple except the darkest ones.

---

Ann

Hi Ann, I hope you re having a good summer. I’ve been trying to understand the "Praia da Ursa, Cabo da Roca, Portugal during the blue hour” photograph in terms of the RGB model and this website https://icolorpalette.com/color/twilight-blue.
Hex #0a437a with Rgb (10,67,122) seems similar to the Praia da Ursa photograph. It has a b value around the mid-point of 0 and 255. I would be interested in your perception of the blue of blue hour. Thanks, Peter

[Ann]

Ann, that’s very interesting to see the purple color with rgb values of r = 123, g = 0, and b = 222. The b = 222 is a high value -- approaching the maximum 255.

I am wondering how to describe the blue in the upper atmosphere behind the purple clouds in the first photo. It looks as if it is during blue hour. Do you think blue rgb values of blue hour would be lower than 222?

I'm not sure that the picture taken by Eiji Ogura was taken during the blue hour. It might have been. I think it is more likely that the other picture was. The clouds reflected the colors of the reddened Sun after it had sunk below the horizon.

Wikipedia shows us this cloudless picture of the blue hour:

Praia da Ursa, Cabo da Roca, Portugal during the blue hour. Credit: Rnbc

---

My guess is that the b value of the sky during the blue hour would be 255.


I found an interesting chart showing the hues, hex codes and rgb values for several shades of purple. There is always more b than r in all these shades of purple, and the b value is at or close to 255 in all the shades of purple except the darkest ones.

---

Ann

Hi Ann, I hope you re having a good summer. I’ve been trying to understand the "Praia da Ursa, Cabo da Roca, Portugal during the blue hour” photograph in terms of the RGB model and this website https://icolorpalette.com/color/twilight-blue.
Hex #0a437a with Rgb (10,67,122) seems similar to the Praia da Ursa photograph. It has a b value around the mid-point of 0 and 255. I would be interested in your perception of the blue of blue hour. Thanks, Peter

Well, there is a reason why the B value of the photograph in question is only halfway between 0 and 255, and that is that precisely that it was getting noticeably darker when that photo was taken. I didn't think of that, when I said that the B value of that picture would be close to 255.

To understand what I mean, take a look at the chart of the B values of shades of purple. Consider the values written in red lettering. For a long time the B value remains at 255, but when the hue is getting too dark, the B value drops.

I would guess that it was exactly that fact that made the B value drop way below 255 in the photo we are discussing. By the way, let's take a look at the shade of blue that a hex value of #0a437a produces and compare it with the appearance of the photo. This is the hue I got when I used your Hex value:

███

As you can see, that's a very dark shade of blue. That is why the B value has dropped well below 255. The sky in the photo of Praia da Ursa, Cabo da Roca, Portugal during the blue hour, is not that dark at all.

I wasn't able to find a very good match for the sky color in the picture, but maybe this will do:

███

The hex value of that shade of blue is #64C4FF. I don't know what its RGB values are, but I would definitely say that it has a B value of 255. All right, I googled it, and the RGB values for #64C4FF are R = 100, G = 196 and B = 255. Only saturated hues display the values 0,0,255 (or 255,0,0 or 0,255,0), and this shade of blue is not saturated.

I actually wasn't able to find a hue whose RGB value is 0,0,255. The closest I got was these two:

███ (0, 11, 255) and ███ (10, 0, 255)

The most saturated green hue I could find was this one (and this is actually all green!):

███ (0, 255, 0)

A saturated shade of red is easy:

███ (255, 0, 0)

Hope that helped.

Ann