Don't worry about this now. Knowing the name of a color at 2.5 is no test of abilities. Don't correct him. They're just colors.
There are tests for color blindness that are more accurate - seeing a shape in a mass of "bubbles" of different colors is a common one. For older kids and adults, they use numbers. For little ones, they'll probably use shapes (tree, boat, etc.). But remember that many people with color blindness can see some colors and not others - my father was like that, and so is my son. It's not like the whole world is gray.
My son never tested as color blind and we only found out much later (in high school). The only adaptation we made was that he asked the teachers not to use both red and green markers on the white board at the same time - they made the letters "jump" around. So the teachers cooperated.
He wore a lot of black and gray clothes so he didn't have to think about it! Otherwise, he hasn't been shortchanged at all.
So yes, mention it to the doctor before you go in for the check up, but don't sweat it. Honestly, what are you going to do about it anyway? He'll adapt, and so will you.
I wouldn't be correct a 2.5 year old on colors, or letters, or shapes, or anything else. Just let him be a little kid and don't push the academics.