There’s not much written about this, Sarah, but your pediatrician is absolutely right. Children seem able to learn the names of colors later than they learn the names of shapes. Notice I underlined the word “names,” as those labels are totally arbitrary and are different in different languages. Knowing the names for the various colors is totally different from distinguishing between different colors. The inability to recognize the difference between various colors is what characterizes color-blindness. Now obviously, if you can’t distinguish between two colors, you can’t learn the names we have given them (or else you do it by learning to pick up subtle clues that would be well beyond a 2-year-old). You mention his inability to “pick out the blue one from the red one.” Confusion between red and blue occurs only in total color-blindness, which is rare. The most common type of color-blindness is red-green.
As soon as your son knows what “same” and “different” mean, you can conduct your own test to determine whether he is color-blind. Hold up two little trucks, one red and one green, and ask, “Are these the same color?” But don’t stop there, as children tend to answer in the affirmative. Ask, “Are these different colors?” Do that with a variety of objects (blocks, T-shirts, socks, etc.), but don’t make a nuisance of it. If he says they are different, then label one and say, “This is the red one. You want to play with the red one? I’ll play with the green one.” Keep up the labeling without making too big an issue of it, and he’ll soon know the names of the colors. Finally, don’t worry that the other children in his playgroup seem to know them. I’ll wager that your son knows some things they don’t know.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.