Fun & Easy way to teach the colors using M&M's
Give a M&M to your kid, by telling the color of that M&M. Once, he eats that give a different color M&M and tell the color of that M&M.
Repeat this process for 2 or 3 days. Then tell him to pick Red color M&M etc, see whether he picks the right color, if he does give it to him. If he doesnot pick the right color tell him to pick another one or give a hint.
Repeat this process. Soon, he will be identifying the colors.
In the begining, try with 3 or 4 colors and then you can teach him more colors.