Use different color papers to wrap empty tissue boxes, and then ask him to put the same color of blocks/balls into the box and also tell him what that color is, he can then learn through playing game
Don't worry. My daughter was also not good at differentiating colours when she was small, although she was very good in speaking. She just gradually learns when she gets older. It's very true that it just takes time. Regarding interviews, don't worry again, differentiating colours is just a part of the interview.
And one more thing, my daughter will soon turn 4. She has been attending painting class in the past year. I saw her mixing black and white to get grey. When your child gets older and is capable to paint, he/she will be able to grasp the concept.