The idea is not mine. It came from my two nieces, A and M, two cutest girls in the world. They were like two mermaids, wandering along the shore and diving into the pearly blue sea. And getting seashells. One day they showed me how to use shells for mixing colors.
I’m sure your kids enjoy searching for seashells, too, when you are at the seaside. Let them use the shells as a painter’s palette. Let them squeeze tube watercolors into the shells, mix them and make new colors from the existing ones, learning about the vast array of subtle color shades.