Colors and flash patterns were established to distinguish one lighthouse from another when seen from far out at sea.

The life of mariners literally depended on accurately knowing which lighthouse they were viewing and being able to pick out the lighthouse from encroaching civilization -- with all the electrical lights that appeared in the 'background' sometimes determined the color of the light.

The light at Fort Gratiot MI is green because it does mark the starboard side of the channel as ships are 'downbound' into the St. Clair River.

Others?