I remember someone was looking at ways to do this a long time ago..
don't remember if they succeeded or not.
Pretty cheap too !! although they will extent the end of your tap :\
"Running the tap is pretty dull, isn't it? You turn the handle and water
runs out. Whoopee-doo! But what if your tap could spew water that looked
like radioactive mouthwash or alien blood? It can if you buy a Glow Flow
Totally bonkers but strangely practical, this neat little gizmo uses
internal LEDs and a temperature sensor to turn water either blue or red.
When the water is cold the Glow Flow illuminates the stream in a cool
electric blue; when it's hot (32 degrees and above) it changes to a warm
red. So as well as looking pretty phantasmagorical, the Glow Flow serves
as a handy temperature indicator to help prevent kids scalding themselves.