J.N.
OK, there are a few types of color blind, most of them are hereditary. So if no one in either family is colorblind it's a lot less likely.
More importantly, what he's doing is 100% NORMAL for a 3-year-old. The names of colors are actually quite arbitrary ("red" has nothing to do with the appearance of the color except what we've assigned to it) and harder for kids to remember than some other concepts. I've worked with preschoolers and taught kindergarten for several years, and I've seen a lot of kids who are still working on getting all the color names right. So it sounds like he's just fine.
(The most common colorblindness is red-green, which tends to usually affect boys. If you are concerned, give him a pile of blocks with about 3 or 4 colors, including red and green, and have him help you sort them by color, without naming the color. If he hasn't sorted before, you'll have to help him get the concept, but then you can see if he regularly mixes up red and green items or if he can differentiate between them.)