Have your child tear various colors of tissue paper into large pieces.
Brush white glue thinned with water onto the back of each piece and arrange them on a piece of white poster board.
Show your child how to create new colors by overlapping two pieces of different tissue paper; yellow over red makes orange, light blue over pink makes purple, and so on.
To give a nice sheen to the finished product, coat it with an acrylic polymer using a synthetic paintbrush (available at an art supply store).