How a red grouper fish uses color signals like traffic lights?

Red grouper fish.

Sure enough, the red color of a red grouper fish is impressive.

But what many do not realize is that the red grouper fish uses it's red color as signals, somewhat like we use traffic lights.

When the red grouper is hunting rather than merely keeping watch over his territory, his color turns a darker shade of red.

The clown fish, which is preyed upon by the grouper, can tell from the grouper’s color when he is ‘off duty.’ 

Picture of a clown fish.

During these safe periods, a clown fish will boldly chase away a grouper who invades his territory.