Although I can not recall exactly when both of my kids learned colors (definitely well before 3 - DD is 2.5 now, although in the 3 year old room that DS was in, they do a "color of the week" theme every week, and not all kids knew them yet), from what I can recall, DD learned colors (and more of them) at a younger age than DS. And that was partially because it mattered a lot more to her - she cared about the color of her clothes and objects (by 2, I stopped trying to put her in red shirts because she would scream "No red! I don't like red!" - for some reason, red provoked that reaction the most then). She learned early that if she wanted to control what color markers/crayons she got and so on, she better know all the words for them. DS could care less what color his clothes are, as long as it was with graphics or patterns he liked. So it was not until later that he started to make it clear that he knew a wide range of colors.
Of course, that meant from a very early age, we have backseat drivers chiming in (it is hard to explain to a 2 year old why you can turn right on red sometimes, but not other times...)
