To keep this mess free I poured a bit of red paint and yellow paint into a ziplock bag. I made sure it was sealed well. Then I folded over the seam and used painters tape to keep it on the table.
Bear kept trying to pick up the bag so I added more tape on all sides.
You can see the orange starting to form. For older children, you can mention primary colors and secondary colors. Red + Yellow = Orange. For Bear, I just talked about the three colors. I also showed him a few of his trains that matched the colors he was making/using.
An additional activity you can do with your child is to "write" in the paint with your fingertip.
Later on in the day, I had Bear make shapes in the paint, but you could easily work on letters or words with older children.
Since the paint is already in a sealed bag I'm saving it for another activity to work on with Bear at a later date (come back for that!). No sense is wasting the art supplies.
What mess-free activities do you like to do?