White light is a mixture of all colors. When an object absorbs some colors and reflects others, we see the reflected colors and thus a certain shade. Prisms and gratings can be used to observe the different colors that are present in white light. Milk and shallow dishes.
Paint works fine, but food coloring is cheaper and allows for more experimentation.
Prepare bottles of colored water to mix. Experiment to find what solutions work best. The light in the room is also important. Adding food coloring to milk in a shallow dish is also interesting. View things through colored plastic film.