We used a $2.99 salad spinner from Ikea - can't beat that for cheap! (My salad spinner doesn't have any drainage holes on the bottom so this was actually a fairly clean project, but if your spinner has drainage holes, be sure to put it on a cookie sheet or something that will contain the mess!)
Cut a piece of paper to fit the bottom of your spinner (stick it to the spinner basket with a piece of rolled up tape to prevent the paper from shifting around while spinning), pour in some paint, and give it a spin!
He's a little young to understand the whole concept of primary and secondary colors but, just for fun, we experimented with mixing primary colors to see what we ended up with when they got mixed together. Great project for preschoolers! (And yes, his cardboard house doubles as a drying rack for art projects. Convenient, eh?)
The paint we were using was pretty thin so we didn't get any awesome results, but we had fun and I was able to use a few of the pieces to make Abe a nameplate for his bedroom door. I half expected him to tear it down by the end of the day, but it's been up for almost a week now and he hasn't touched it. I guess he's finally learning to recognize a masterpiece...and probably wondering why I didn't put a velvet rope in front of it. ;-)