Additive Light mixing
This is the classic 3 circles of light exibit that you can mix to make other colours. All three lights have brightness controls, so visitors can change to colour of light to anything they like. The lights are also moveable, allowing more interaction.
This exhibit has recently got a lot easier and more practical with the recent improvement in LEDs, which give a clean very saturated light. It can now be made to work in a normally lit room (though not in direct sunlight).
This exhibit goes particularly well with subtractive light mixing.
The primary colours of light are of course to do with human biology, and the colours that our cone cells are sensitive to, a different animal (or a colourblind person) would have different primary colours.