PHET Description:Make a whole rainbow by mixing red, green, and blue light. Change the wavelength of a monochromatic beam or filter white light. View the light as a solid beam, or see the individual photons.
Teacher Description: if you are a science teacher you are probably familiar with PhET Simulations. The website has many sims for things like matter, waves, and molecules that are help students visualize the abstract or very tiny. This sim is good is appropriate for teaching students how color filters work, and demonstrates that white or colorless light is made up of all colors.