One of my favorite preschool days so far was one of the days that we worked on colors. (Now I have to admit, Jack already knows his colors pretty well, so some of these activities may have to be adapted or more guided for kids who don't know their colors at all yet. But it's good practice and reinforcement, plus great for counting/sorting by color.)
First we read The Dot by Peter Reynolds. If you haven't read it, you should! I love this book! It's a cute, fun story about how we all have the ability to create. I think the message was a little over his head, but he liked the idea of painting dots in lots of different colors!
So that's what we did next. And by painting dots, I mean he got paint on his paintbrush and rather aggressively jabbed his paper with it, which left more splats than dots. But every time, we would practice saying which color paint he was using, and he would yell "dot!" when he hit the paper.
Then I got out Twister! I had basket with a bunch of small toys and other objects that matched the colors on the game (toy cars, legos, cookie cutters, pipe cleaners, paint chips, etc), and told him to put all the green toys on the green circles, the yellow toys on the yellow circles, and so forth. He actually caught on faster than I thought he would, and loved this new "game!" And of course, being the somewhat OCD child that he is, he lined up all the hot wheels cars in a row, all the pipe cleaners in a row, etc. He cracks me up.
We played another more active game on the Twister board too - he would stand at one end, and I would tell him things like "jump on all the blue circles" or "run on the red circles." Not surprisingly, he loved this game even more! And very quickly insisted that "Mommy run too!" So we ran and jumped and raced on the different colors for a while. We would also count the circles as we stepped on each one (except when we were running of course!)
And to finish it off, for part of his snack he got m&m's - little colored dots! So we sorted and counted them by color as he ate them.
There are so many other activities you could do with this theme! Learning, sorting, and finding colors is so much fun for kids this age - just look around your house and see what you have. Anything from crayons to paint chips to fabric scraps will work. Be creative, and most of all, make it hands-on for the kids!