The other day, I realized a way that a blind person could tell what colour something is. This is not a particularly useful invention, but it is nonetheless a neat idea. The basic premise is:
Four lights are attached to a device in a line. One emits white light, another red, another green, and the last blue. The device would be set against an object and turning the lights on for a short amount of time. Then, somebody could feel the four spots with their fingers. They could tell how dark the object is by how hot the first spot feels, and what colour it is by the heat emitted by the other three spots.