I use a paper plate and put two primary colors touching on the plate. I use a paper plate because it sort of absorbs the paint blobs. (When Leila turns the plate over the paint stays on the plate.)
For our St. Patrick's Day shamrocks I used blue and yellow. On the other side of the plate I gave her a bit of green and white paint. Again, the two colors are touching each other to make it more likely she will mix them on her own.
You can paint with paint brushes, your fingers, sponge pieces, plastic forks, anything! We used Q-tips and they were the perfect size for her little hands. Leila loved making dots and circles. (It's so cute how she says "circle".)
You can see how she naturally mixed the colors without making a brown muddy looking mess on her paper. The color selection is key!
I found a shamrock pattern online because it was easier than trying to draw one the was good enough. After I cut the pattern out, I traced the shamrock on the back of the dry painted paper. (No pencil lines on the front!) I wrote Leila's name and the year on the from of each shamrock in a matching color.
Ta-da! All ready to be sent to the relatives for St. Patrick's Day.