According to a blurb in Science yesterday, researchers have discovered a fossilized fish whose eyes show traces of pigment and also fossilized rods and cones. The existence of the cones suggests that color vision developed at least 300 Ma ago. You may read the full article, which appears in Nature Communications, by following the link from the Science article; you can read it only on screen – a pdf will cost you $32.
P.S. Yes, I learned about Nature’s sharing policy by tracing the link from Science. If you follow the link to the Nature article itself, you get only the abstract.