ColourLovers has a collection of very colorful nudibranch photos, and their color palettes, too!
Part of the mollusk phylum, Nudibranchs are the shell-less relatives of the snail and are known for their garish colors. These tiny sea creatures are usually only 2cm - 6cm in length and can be found worldwide. They are able to thrive in any depth of salt water from the deepest darkest ocean floors to warm shallow water.
Link -Thanks, Lex!