red and green should never be seen…

An old phrase with various meanings; one such refers to the running lights on a ship - red lights on the starboard bow and green lights on the port bow.  

The phrase then becomes; red and red or green and green - but red and green should ne’er be seen.  It implies that if you can see both red and green lights on a ship at the same time, then it’s heading straight for you.  Which is not good.

Luckily this rule does not apply in photography.