This isn’t just a fun thing to have…it actually tells you how hot the water is before you step under it! Green lights indicate below 90°F, blue covers 91 to 105°F and red is about 107 to 113°F. Anything above that and the shower head will flash red to let you know you’re going to scald yourself if you don’t add some cold to the mix.
An internal generator is powered by the flow of the water, so no worries about having to run power and water together. You can pick one up for right around $13.00 US.
Source: Oh Gizmo!