Teaching colors can be challenging, but it also can be FUN!
When my son, "Johnny Apple", was about 2 years old we would focus on recognizing and saying our colors.
To help increase his exposures to colors and record his progress, I made an interactive poster for in his room. I just got a regular poster board from the dollar store and drew and colored in the crayons. Make sure you write the color's name by the crayon to help him recognize it's name as well. Then, everyday we would do our colors. I would point to a color and ask him it's name, he would respond "Green" or "Blue"etc.. But, I would also change it up by asking "Where is Purple?" then he would have to point to the color. Each time he did either correctly I gave him a sticker to put on the crayon or by it. He LOVED doing this. I loved it too because I would know, just by looking at the chart, which ones he needed more focus on.