Faucet that changes the color of water according to the temperature of water "Light Delight"
It does not change the color by mixing the coloring matter, but a mechanism that changes the color by lighting the water with the LED light. If it is cold it will be blue, and as it gets warmer it will shift to red.
You can understand well by watching the movie of the site below.
A movie in which the color varies with temperature
Product site is here.
As a similar item there are the following things.
LED Faucet Light - Gizmodo
Hansa's temperature sensitive faucets - Engadget
I wonder if Japanese water supply makers will soon adopt it.