I took a glass casserole dish and squirted shaving cream all over (I just bought the cheapest shaving cream they had at Wal-Mart) then smoothed it out with a butter knife.
Then I dropped blue and red food coloring into it in various spots as shown here.
Then I gave each of my boys a plastic fork and they pulled the colors through to mix them as shown below. You could also use toothpicks to distribute the color, however you want to be sure you don't mix it so completely that the individual colors are obscured.
After mixing the colors, gently place your paper or cardstock facedown on top of the shaving cream then carefully lift it up.
The next photo isn't great, but I was holding the paper covered with cream in one hand and trying to snap the photo with the other hand. You can see all the shaving cream glopped onto the cardstock.
The next part is the coolest part. Using a straight edge (I used a wallpaper thing; I think you could also use a ruler or whatever you have), scrape the shaving cream off. This leaves a marbled look on your paper. I was really excited about how it turned out since I've never done this before. I just took the scraped off cream and threw it away in a paper towel. It didn't make sense to me to add it back in with the rest.
Below you can see one of our finished pieces. We just painted 2, one for each boy, but I think you could make a lot more using the same shaving cream mixture.