[GoldenRacer]

Would they be able to distinguish between a light that is pure orange (a single 600nm wave length) and a light that is a mix of red and yellow that gives humans the perception of orange?

If so, I think that's still kind of a cool thing to think about.

[shrx]

It is my understanding that a mix of red and yellow frequencies would trigger the corresponding yellow and red-sensitive receptors, if it has them, but not orange.

[rojobuffalo]

That's my impression as well.

It's an interesting question. If you want to read more, the two researchers quoted in the book that I was summarizing are Justin Marshal and Mike Land. Each has a handful of papers that are cited in the bibliography.