Q: Did you hear about the red ship and the blue ship that collided?
A: All the sailors were marooned!

Explanation: Marooned: to be stranded on an island after a ship wreck.  Maroon: a dark red, almost purple, color.  So, when red and blue are mixed, you get purple, well, maroon.  This joke plays with the knowledge that red and blue make maroon (almost, sort of maroonish purple) and the knowledge that a boat crash may leave sailors marooned.