When we painted in the past, I put the paint on small paper plates. We seemed to be going through a lot of paper plates, so I tried to come up with a more resourceful way to go about this process.
We got some cheap tins from the dollar store and turned them into the kids' individual paint trays. I now have room to place many colors in this tin and the kids can do what they want with the colors. If they want to mix colors, they can. If they want to use them separately, there is plenty of room to avoid mixing.
I don't even bother washing out the tray when the kids are done. (The tin is becoming a masterpiece now too :) )