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

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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?

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