Relationship Between Eye Color and Color of Sun

Question

Does anyone know what the relationship is between the range of likely eye colors in animals and the color of the sun on a given alien planet? For example, on Earth we primarily see browns, greens, blues, and some yellows for eye colors. Would animals on a planet orbiting a red dwarf, for example, tend to evolve with different eye colors?

Answer 1

The simple answer: No.

The predominantly brown colouring you see in human eyes is the result of brown pigment molecules in the iris. Changing the Sun's colour won't change that.

The less simple answer: Maybe.

• The colour of an object is just the spectra of light "reflected" by that object. Change the colours available for reflection, you technically change the colour of all objects in that light.
  • We don't normally perceive this as being the case because of something known as colour constancy. Our brains work to adjust our subjective experience of colour to give us the illusion of consistent colour under different lighting conditions, rather than different colours with different lighting conditions. Therefore, if we evolved under radically different lighting conditions, it's conceivable that we would perceive colours differently, rather than having brains that worked to adjust colour experience to what humans on Earth consider a normal colour range.
• Blue eyes, unlike brown eyes, aren't coloured from pigmentation. They exhibit structural colour. That might sound a little strange at first, but it starts making sense when you remember light has wave-like properties. Every "colour" actually has a unique wave length, therefore you can use geometry (on the nanometre level) to control what colours go where. In this case, the specific effect in the eye is Rayleigh scattering. Blue light is scattered more than other colours, so we see more of that.
  • Under a star with far less blue light than our Sun, you wouldn't see blue eyes as blue. But the consequence of radically different light on blue eyes doesn't end with blue eyes. All eyes are actually blue but with differing amounts of black-brown pigment on top. So blue eyes are actually just eyes with fairly little pigment, and other eye colours humans normally have (green, yellow, etc) are the just the result of different amounts of brown on top of structurally blue irises. If you changed the natural lighting enough to make blue eyes not blue, other lighter eye colours would be affected too. (There would probably be a minimal structural colour effect on the darker colours, not just the dark browns.)
• It's worth noting that while different types of stars do output different spectra of light, the stars that can conceivably host a human-habitable planet put out light that's fairly similar in colour. (The effects would be minimal.) In addition to carefully picking the kind of star you want for your world, think about the atmospheric conditions of the planet in question. You probably want to think about mechanisms to block out certain ranges of colours, since hot things don't usually skip colours. They just skew a bit into the red direction or into the blue direction. It's a detail that won't concern your readers or players very much. In fact, you may not even explicitly explain what colours are screwed with, just what the results are. But it would at least provide a hint of plausibility to people with some physics background.

Answer 2

No. The color of the structures making the iris don’t have anything to do with the light profile it’s used with. If there are protective pigments there is something: adding something to block UV (which can damage organic molecules) may happen to be some color. But with a red dwarf sun lacking UV, there is no need to add pigments. So you may see uniform appearance in the population with color that is the natural coloring of the muscles or other tissue involved. Consider for reference a terrestrial albino.

Answer 3

There is none. Eye color is almost completely accidental.

Humans are thought to have very distinctive iris colors, contrasting with the pupil and sclera, for social reasons-because it makes it easier to tell where someone else's eye is pointing and track their gaze-but the range of colors available is a completely accidental result of mixing pigments that our bodies already created for completely unrelated reasons into different layers of the iris's muscle structure.

If you had a situation where eye color served a more complex signalling function within or between species, then maybe you could get some actual evolutionary pressure towards particular colors, but in that case it would be constrained by what colors the creatures in question can see. That's why mammals tend to be drab while birds tend to be brightly colored, for example- because most mammals can't see a lot of color, so there's no point being colorful, while birds can see color very well, so being colorful is useful for intraspecies identification. Eye colors will thus be driven ultimately by the ancestral lifestyles of the creatures whose eyes they are, and not by the color of the sun.