Hey, Relevant XKCD time!

The sky is blue. Why, you ask, is the sky blue? Because air is very very slightly blue.

You could ask why is air very very slightly blue, and the answer would have to do with Rayleigh Scattering, and probably some other effects.

You’ll get a similar, but slightly different response with water. Water’s color mostly has to do with the absorption of certain frequencies of light, whereas air’s color has to do primarily with the deflection of certain frequencies of light, but at the end of the day, both substances interact with light in a way that preferentially funnels certain frequencies of light away from them (and towards the observer).

Now, why is this relevant?

Because “color” is a word we use to describe an experience, or perhaps an observation, rather than a physical property. It’s not “real” in an empirical sense, since what we’re experiencing or observing is light that reflected from the object, rather than some physical property of the object itself.

Now, that’s not to say that color is all made up; a red cube does, verifiably, emit photons around 650 nanometers in wavelength. A violet cube will emit photons around 400 nm. Well, what wavelength does a magenta cube emit?

Trick question. It emits both 650 nm and 400 nm. It’s not a color on the visible light spectrum, it’s our brain’s interpretation of those two simultaneous signals.

So, while objects may absorb other frequencies of light, we describe their “color” by what they emit, or cause-to-be-deflected-into-our-eyes (depending upon the specifics of the mechanism, with Cherenkov Radiation being an extreme example).

Now for the fun stuff. As discussed above, air is very slightly blue, but that’s only if your eyes can see blue. Just on the other edge of infrared from visible light is something called the Terahertz Gap. It’s an area of electromagnetic radiation that penetrates most of the materials we construct things out of (paper, wood, clothing, plastic, ceramic, etc), but is quickly absorbed by air. It’s very close to the frequencies used for millimeter wave scanning, like in airports. Because things like cardboard are invisible to it, it can be used in automated manufacturing processes to make a camera that inspects items coming off an assembly line after they’ve been packaged. This is especially helpful in electronics manufacturing, because you can photograph the inside of a computer chip.

When we say something is a certain color, we’re talking about the color reflecting off of it. It’s color is the color that actually reaches our eyes.

Mm this one doesn’t work for me. Why is an object the colors of light it absorbs rather than those it reflects? Neither is instrinsically preferential. However, if an object is red, it is reflecting ONLY red light. Therefore it is doing something special with RED light. I’d say that breaks the tie and the object should be considered red.

And what color is a stoplight?

Colour is an illusion, it’s a bandwidth of visible light within the spectrum. Colour does not exist in nature technically without the eye to perceive it, just another electromagnetic wave

You see the colors you do because —

• As an ape and a mammal, you have eyes that have evolved to make distinctions among those colors, e.g. to tell red fruit from green leaves, or orange tigers from brown horses.
• The earth’s atmosphere is mostly transparent to those colors (frequencies of light). Chemicals that are opaque to those colors tend to get burned off from the atmosphere by sunlight.
• The sun emits most of its energy in those frequencies. This both ① burns off the opaque chemicals in the atmosphere, and ② means those colors are the best for an animal to use to see things.

Your eye was not “designed” to see red, but being able to see red is a good trick for an ape that sometimes eats red fruit or red meat.

The earth’s atmosphere was not “designed” to be transparent to certain colors of light, but any chemical that isn’t transparent to a frequency of light will absorb energy from it and thus tend to burn off.

The sun was not “designed” for us to see by, but using the available light to see is a great trick for animals of all kinds.

I still like red

@BitterSweet Citations needed; I think you will find the facts are the opposite of what you just said.

Does this work for other colors?