Earlier quoted context omitted.
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.
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.
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.