Traffic light in an Erlenmeyer Flask
by David Ng
A solution of glucose, sodium hydroxide, and indigo carmine, when shaken, will change from yellow to red to green. Left to sit, it will revert to red again, then yellow, and the process can be repeated. The indigo carmine is green when oxidized, yellow when reduced, and red in the intermediate semiquinone state
From Futility Closet, and available for purchase at Ward’s Natural Science.
popperfont.net 
I like the idea, conceptually, but the energy required to shake the solution (and possibly still have to back-light it) would likely trump the energy required to power modern LED-based traffic lights.