To do this activity, I squirted red paint in one side of a gallon zip lock bag and yellow in the opposite corner. I sealed the bag and taped it to the table.
The girls liked mashing and squishing the bag to mix the colors.
O(6) eventually took the tape off the bag so she could roll it and squish it all around.
She wrote messages on the bag for me to read.
They both loved this activity, and I loved that it was so simple and there was no clean-up! The idea for this activity came via Pinterest, various blogs, and children's craft books...it's an oldie, but goodie :)