A bit of fun I had when building my subway module was figuring out how to illuminate subway signals. I ended up using fiber optic lines fitted into Shapeways 3D-printed subway signal bodies, with colored bits of plastic over the lens, all powered by a 9-volt battery.
An unanticipated twist was that the light source (an LED) had a bluish tint to it, making it difficult to get reasonably accurate colors: green plastic looked yellowish, yellow plastic looked greenish, and red plastic looked purple. So, orange became the new red, yellow became the new green, and green became the new yellow!
