In the meantime, he'll add new colors to his repertoire through practice. Children love looking at picture books of objects organized by shape and color. Start by asking him to identify things nonverbally. For example, ask him to show you a red square then let him point to it.
You can play similar games when you're out and about. Say, "I see a red flower," then wait a minute to see if he points to it first. If he's wearing a blue shirt, ask him if he sees anything else around him the same color. He may surprise you by knowing and identifying colors you point out, even if he can't name them verbally.
As he begins to learn the names of the colors, you can reverse the game, pointing to objects yourself and asking, "What color is this triangle?" Either way, he'll delight in showing off his knowledge. When he misses one, don't tell him he's wrong (or pretend he's right). Just say the correct name in an encouraging tone.
Kids learn at their own speed, so don't be too concerned if your child doesn't know as many colors as someone else his age. But if you suspect a problem, talk to your child's doctor about whether you should have your child tested for color blindness, which is the inability to distinguish certain colors. If necessary, the American Optometric Association recommends getting your child tested before he begins school.