Cut dark-coloured paper to fit a shallow tray. Prepare four colours for painting. Place in each pot of paint a large marble and a plastic spoon.
Start with the lighter of your colours first, e.g. yellow. Spoon out the marble onto the paper in the tray and encourage the children to move the tray slowly so as to make the painted marble leave traces of colour.
Spoon out the marble and place it back into the paint pot, ready for the next child.
Repeat this process with all your colours.