About 1 in every 2 billion American Lobsters is blue.
Blue lobsters are quite rare, and their coloring is actually due to an odd genetic mutation. This causes the lobsters to produce much more protein than necessary. This protein combines with a molecule and forms the lobster's blue coloring.
Yellow lobsters are also quite rare. In 2006, a lobsterman named David Percy caught a yellow lobster. Apparently, the odds of this are 1 in 30 million.
"On July 13, 2006 a Maine fisherman named Alan Robinson caught a half-and-half lobster, where the colors are perfectly divided on each side of the shell. He submitted the brown and orange lobster to the local oceanarium which has only seen three lobsters of this kind in 35 years. The chance of finding one is estimated at 1 in 50 million. Lobster shells are usually a blend of the three primary colors: red, yellow and blue. The colors mix to form the greenish-brown color of most lobsters" (Wikipedia).
However, all of these split lobsters were hermaphrodites. :D
