To make the colored ice cubes, I filled an ice tray with water and added food coloring (a lot of it!) to each cube of water before freezing the cubes. We painted on white cardstock since the paper needs to be thick in order to prevent holes from all of the water.
O loved painting a sheet just one color until the ice cube melted, and E loved painting with all of the colors. I also really enjoyed painting with the ice cubes, and I was surprised by how well it worked (we did this when O was almost 3, and I don't remember it working this well).
O said this was her favorite art project that we've done all summer (we've done a lot...I just haven't found the time to write about them yet). She and E would have painted all night if the ice cubes hadn't melted. Luckily, I refroze them so we can use them another day!
Btw, our hands are stained, but I think it will wear off in a day or 2.