HANSA International's Hansacanyon Faucet has LEDs that light up with different colors based upon the temperature of the water passing through it. Blue is for cold water, violet for lukewarm and red for hot.
These have been around for years. If found nowhere else, local science museums usually sell them as an add-on spout for around $25. A quick Google... http://dx.com/s/faucet+led+change.html?sort=Price
Looks more like the water changes color.