Subtractive light mixing
If you are mixing light you add colours together, and if you get the right mixture it will look white. When you mix pigments you start with white light, and remove colours. So the primary colours instead of being one of red, green or blue, they subtract red, green or blue, leaving cyan, magneta, or yellow.
This means theat if you mix red with green or blue with red, no light can get through, which can surprise a lot of people.
The exhibit itself is a simple light box with a variety of coloured filters to try mixing on top. It is much more effective in conjunction with an additive light mixing exhibit.