vivid and bright colours do not diffuse into one another as they are mixed, to allow different magical effects.
paper, cardboard, Bristol board, plastic ...
Blue + red = purpleyellow + blue = greenyellow + red = orange. A feature of poster paints is that they can be mixed, and your kids know it! Now surprise them by giving them Prima Magic. This paint is formulated so that the colours do not mix, and will astonish them with the unexpected beauty of the effects when they attempt to mix them!