Hi M.:
Boy have I been through this!
My brother, who was 15 months older than I, was colorblind. So, I grew up with it.
My son, who is 10 years old, is colorblind.
AND, I think my 3 year old son is as well. I'm going to give it a little more time before I'm sure.
When my 10 year old was 5 I became sure that he, indeed, was colorblind. He mixes his colors. Brown with Green, Orange with Green, Pink with Orange, Green with Black. Just like my brother did. Now I am seeing the same color mixing with my 3 year old.
You can count on the usual colors (listed above) for confusion.
Yellow, red, blue, white, black are all pretty consistently correct.
I hope this helps! You will probably find that it really isn't a huge deal, unless he tries to dress himself!!!! As long as you make his teachers aware, he won't have many problems at school. Once the other kids find out, they will ask your son on a regular basis what color things are, just to see him get "that look" in his eyes (you will find out what look that is when your son gets older. It's a kind of questioning, concentration and quessing look. Very cute, I think). Anyway, the kids love to ask him what colors are. It gets a bit bothersome, but not enough to have made my son or brother upset.
I hope you will find it to be an endearing quality about your son.
Good luck and don't dress him in pink when he's a teenager. He WILL eventually find out!!!!