Fictional Color: Proliferate, since it is the nature of exotic species in general that not all of them will use the same chunk of the electromagnetic spectrum for seeing with. Most obviously, if you have typical eldrae eyeballs, before red in your spectrum comes calescent gallé, which we can only feel as infrared, and beyond violet are whiter-than-white iven and naught-but-bright séris, extending into our ultraviolet and making many colorless liquids anything but. But perhaps they do not count, since we could theoretically create them – just not perceive them directly.
But then there are those species with different sensory organs. The retina has many fascinating quirks, but one of them is that it can’t tell the difference (for reasons explained here) between monochromatic yellow light, and yellow light that’s actually red light and green light mixed.
Not all photoreceptors share this particular property. So if a species you’ve never met before asks you to push the green button, you’d better be sure to check that you aren’t accidentally going to push the blue-and-yellow one, which is obviously completely different, dammit, Earthling!
And that, of course, is before you even get to the wonders virtuality programmers can create with the ability to stimulate the visual cortex and its subcarriers in ways that physical light cannot, letting one actually perceive wild and painful ulfire, dreamlike, voluptuous jale, and other such colors whose qualities aren’t found in electromagnetism. Irrigo, violant, apocyan, cosmogone, viric, and pelegin may also be on the menu of options. Even octarine is not beyond the bounds of possibility…
A post I made elsewhere concerning colors according to alien eyes:
Indeed so, even if they happen to use the same chunk of the EM spectrum we do as “visible light”. There are, I suppose, some restrictions on what’s likely, since moving too far into the longer wavelengths starts to give you resolution problems and moving too far into the short wavelengths gives you issues once you start trying to come up with plausible eyes that run off ionizing radiation, but even so. The eldrae, for example, whose visual spectrum stretches a little down and a little further up from the human one, have three colors we don’t: gallé (infrared), ivén (low UV), and serís (high UV); the kaeth, by contrast, have two infrareds, no UVs, and can’t see the wavelengths we call “violet”, either. And that’s before we get into all the subjective issues of how the actual qualia differ, if that’s even a question that’s possible to answer.
The standard interlingua has a set of compromise color names defined by the frequency bands of the light in question, specifically to help sort out this whole sort of general mish-mash with which colors look like what to whom, once all the optical system differences get sorted out. (Consult your friendly translator’s test card for what each color looks like to you!)
Of course, where life gets really interesting is that the human visual system, due to its architecture, innately does some averaging inasmuch as it can’t tell the difference between red-and-green and yellow in light, or between blue-and-yellow and green in pigments; the brain never gets that data. But there’s no reason that a visual system has to work that way, although I assume in my universe for various implementation reasons most of them do – so, imagine how wildly different the world would look to someone who sees all the different mixed pigments/lights we use to produce that shade of green as entirely different colors, and the same for every other shade…
(And then try and imagine using one of their control panels – quick, is that the yellow alarm light flashing, or the red-and-green one?)