I came across this idea recently and decided to try it for myself. My girls thought it was fun and I loved how easy it was to clean up after their lasting painting project.
Take a muffin pan and line it with cupcake liners. I used the silver ones so that the paint didn't seep through the paper. You don't have to use the liners at all, but it does make things easier. All I had to do was lift out the liners and wash it down a bit. Sometimes we use those styrofoam liners that come under fruits or meats, but the paints all run together. So, if you don't have a fancy paint tray, this is the way to go!