Why do some houses have green porch lights some just red and other a combo of both outside?

The significance of RED and GREEN porch lights or exterior garage lights is a maritime reference to port and starboard lights of the ship. Sailors place red and green lights on their home as a significance that they are 'Captain of their ship' ie, their home. Or simply put, "It's an inside thing with all sailors"

Additional folklore of the red and green light on display goes as follow; It is said, that when there is a missing sailor out to sea, the dock in which he departed is illuminated at night so that he may find his way home. It is also rumored that wives of maritime husbands illuminate the red light when he is out to sea and the green light when he is home. This red light however, is NOT to be confused with the red prostitution light. Inside the maritime world, a red light on a ship is the emergency signal, or General Quarters lamp. Classic color movies featuring submarine or ship battles often feature the bridge deck illuminated in red light only.