I let him pick the colors and filled each bottle with liquid watercolor diluted with water. Then I placed two coffee filters on an old towel in a cake pan.
He caught on right away and went to work.
It was great to see his little hand muscles flexing. I have never before seen him so engaged in an art project!
He systematically emptied each bottle and then requested to do it all over again. I moved the coffee filters to some newspaper for drying.
We gave them out as decorations. An older child could cut them in the shapes, or I could have cut them in small pieces and had him make contact paper art!