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In a previous question - How might kinetosynthesizing "plants" look? - I introduced a worldbuilding thought experiment of mine centered around a habitable moon, heated by tidal forces, which orbits a rogue planet gas giant.

This planet harbors complex life, including animal-like fauna. Now, Earth animal use their coloration for many things - camouflage, mimicry, and display among them - but these things are only useful if there are other sighted beings around to see you.

On a rogue planet, there would be no need for a sense of sight, because there would be no sun and therefore virtually no light, except during fires, volcanic eruptions etc. My question is; on a moon where no life forms can see, what need would animals have to be one colour or another?

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    $\begingroup$ Color is a sensation; it exists in the mind of the observer. It is not a physical quantity; it does not exist independent of an observer. In a world where nothing can see there cannot be colors. $\endgroup$ – AlexP Dec 31 '18 at 21:28
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    $\begingroup$ @AlexP Noted, but I meant pigmentation; such that were you to shine a light on the creature, it would appear one color or another. $\endgroup$ – SealBoi Dec 31 '18 at 21:32
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    $\begingroup$ What you call pigmentation, the critters of that world call by other names; that it appears to you to have color does not mean that it does not also have some function for them; possibly it changes the texture of the skin, it reflects or attenuates sound waves, it emits odors, it enhances or decreases heat exchange etc. $\endgroup$ – AlexP Dec 31 '18 at 21:38
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    $\begingroup$ @AlexP Those examples together form a perfectly good answer to the question. Perhaps I should clarify: I'm only talking about things which will make them appear colored to humans in brightened conditions. $\endgroup$ – SealBoi Dec 31 '18 at 21:47
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"color" has a lot to do with reflection and absorption of energy. Depending on the intensity of and proximity to your tidal force heat, it might be the case that creatures evolved to absorb and reflect different amounts and spectra of these tidal energies.

While I am not sure that would lead to vastly different "colors" in what we think of as visible, it could/would result in conceptual "color" differences.

Diet might also play a role in creature "color". For example, did you know that flamingos are not naturally pink? Why are flamingos pink

Exposure to other environmental factors might also play a role, such as a potential greening from exposure to copper.

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You specifically said animals

Plants would still need color to achieve the most efficient photosynthesis possible from what little light arrived from other sources. Even if you're rogue between galaxies, the sky is never completely black. So, plants have reason for color even if there's nothing around to see them.

Blind animals, on the other hand, would have none. Color would certainly exist, but only as a byproduct of the chemistry of their skin/hair/fur/etc. It wouldn't otherwise matter.

But there deserves to be a frame challenge

How many of your animals would actually be blind? The number of species on Earth that are totally blind is quite small. Granted, they are all deep-sea or cave-dwelling species where what we call visible light is in short supply, but remember... your night sky is not totally black (unlike a cave or the depths of the sea).

But that's only light visible to humans.

I can easily believe that on a rogue planet experiencing eternal night the development of both ultraviolet and infrared spectral vision is completely believable. Add to this that tri-color vision is pretty much unique to Earth's primate species... including humans (the rest see in duo-color, functionally black-and-white) and it's totally believable that your animals would see just fine.

Another way to look at this is that animals don't necessarily see color (see below). They do see contrast (Human periphreal vision is this way). From this perspective, it's only important that a caterpillar be the same color green of a leaf for those species of birds that see green-and-white (greenscale, and I don't actually know if any bird sees in a "greenscale"), but perhaps it's more important that the caterpillar be the same luminosity of the leaf in question so that it blends in via the grayscale we assume most animals enjoy.

But, let's consider a bit more science...

But, what color does the animal see? Vision, like all of our senses, is processed in the brain. Without being able to get into the head of an animal, it is only possible to know what colors can be detected and not how they "look" to the animal. (source)

In your case, critters avoiding infrared-seeing predators would evolve body heat similar to the thermal conditions of their preferred surroundings. Their skin would have absorption characteristics that allowed the edges of their bodies to blend into their background. We don't think of that as color because we don't see that color spectrum. But evolution would accomodate a predator whose brain can process infrared (or ultraviolet) frequencies systematically.

So, I believe you would have sighted creatures and they would enjoy the color of their world, which would IMO evolve in spectrums that we humans don't see and can't appreciate.

One last note, according to this article, some terrestrial critters have 6 photoreceptors and one butterfly has 15! More types of photoreceptors means the ability to see finer shades of color. Given this data, your moon may evolve critters with very high numbers of different photoreceptors to take advantage of what little light exists. They wouldn't see color the way we do at all and they would likely be completely blind in the strength of our sunlight, but they'd see a world of brilliant — if dim — color.

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    $\begingroup$ Seeing in two colors or dichromacy would not be functionally the same as black-and-white. That would be monochromacy. Dichromacy still allows for the perception of colors. $\endgroup$ – Mike Nichols Jan 4 at 20:02
  • $\begingroup$ @MikeNichols, no it doesn't. You have 100% saturation of one color varying to 0% saturation. There'd be nothing for the observer to compare against to discern anything other than "color or lack of color" - aka, black and white. $\endgroup$ – JBH Jan 5 at 4:07
  • $\begingroup$ @MikeNichols, wait... let me think about that for a moment. Technically you would always be "too bright" (white), "too dark" (black)... and something else. OK, you've got me convinced (I'll update my answer later, I'm out of time). You wouldn't have a reference unless someone identified the color, but if intelligent, you would learn a name for it. Ooooh.... you have me convinced. My apologies for the previous comment. Dang. $\endgroup$ – JBH Jan 5 at 4:12
  • $\begingroup$ @MikeNichols, OK, I took a moment. "An object appears white when it reflects all wavelengths and black when it absorbs them all.." A dichromatic can't see white. They would only see the reflected red. In other words, they really would see "black and white" except that "white" would be replaced with another word. They'd learn that they see that "other word," but they'd never understand "white." Cool, though. $\endgroup$ – JBH Jan 5 at 4:21
  • $\begingroup$ I think you are still confused. If an organism only has one type of photoreceptor then they have monochromatic vision and see in black, grey and white. This is because with only one type of receptor there is no way to distinguish between different wavelengths or frequencies of light. Instead, you can only distinguish between magnitudes. With 2 photoreceptors tuned to pick up different wavelengths (dichromacy) vision is no longer one dimension and different colors can be distinguished. $\endgroup$ – Mike Nichols Jan 5 at 18:15

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