Remember the old rhyme: "I never saw a purple cow, and hope I never see one; but I can tell you, anyhow, I'd rather see than be one."
Is it so absurd to imagine purple cows? There is a certain purple bacterium that lives in salt lakes, called Halobacterium halobium, that accomplishes a primitive sort of photosynthesis using a pigment other than chlorophyll. The pigment absorbs light in the middle (green) part of the solar spectrum. This leaves reflected red and blue light at opposite ends of the spectrum to give the bacterium its purple color.
H. halobium may have appeared earlier in the history of life than organisms that use chlorophyl. It is not unreasonable to imagine that they might have remained the chemical template for all successive plant life, in which case even land plants might be purple.
And if plants were purple, then some animals might have evolved purple protective coloration. Hence, purple cows. (wink)