First stop: pop the top off the Nest button, the combination input switch and multi-colored sweeping light.

An ambient light sensor sits in the center of the button, with lenses in the translucent cover to increase its viewing angle.

A ring of RGB LEDs surround the sensor to make up the Protect's indicator glow:

White for the automatic night light

Reassuring green to indicate all's clear

Yellow for an early warning

Red for a get-out-of-town emergency