My DD was very early to learn her colours but at first she always got them mixed up and most of the time she just guessed. I used to talk about colours constantly at every opportunity. My niece on the other hand is nearly 3 and still can't figure them out but I am not sure how often my SIL talks about colours.

Just keep talking bout them and she'll get it. Praise and clap when she gets a colour correct even if she guessed it.