Draw different-coloured circles on a piece of white paper or cardboard plate. You can also draw more complicated shapes and tell the child to fill their surface areas with different-coloured plastecine.
Biscuit cutters can come in handy for “plasticine cakes”.
Make plasticine balls of different colours and sizes and with older children you can make other shapes: cylindrical and cubic shapes, or even more complex ones, such as animals, plants, human figures, various objects, etc.
Warning: If the child is younger make sure that it does not put the plasticine in its mouth and do not leavve the child unattended.