Light up your…..tap.
Definitely qualifies for the "why didnt I think of it first" category!

Enqua has a new LED tap system which does the simple, obvious and very useful: LED’s embedded in the tap detect the tempearture and light up the water stream red when its hot, blue when its cold, and purple when its somewhere in between.
I just wish I thought up these kind of things!
[Source: Engadget ]