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…