DS has known his colours from about 2 yeh. I would just say the colour whenever there was something he had his eye one.. such as a greeen car. Cars are a good one as you can do it while driving. Toys even. Just a everyday thing. Then after a little while ask her what colour something is like her shoes. If she gets stumped just say, you have Pink Shoes. Then the next day ask the same question.

Best of luck.