What a fun way to break in a new set of watercolours! Make 100 colours. Or 60, in our case, as we only had small watercolour paper on hand. It turned out to be a nicely meditative way to learn about colour theory that was more challenging than I expected.
The kid normally loves colour mixing more than painting itself but he petered out about half way, stating he wanted to finish it tomorrow. Interestingly, though, during clean up, he started playing with various colours in our plastic egg carton spending more time colouring mixing in water than he did on paper. Whatever floats your boat!