Although you might be familiar with adult Red Rock Crabs (Cancer productus) that are primarily red, juvenile Red Rock Crabs come in a variety of color forms (see below). We don't see the form shown above (mostly white with a large brown spot) as often, so I thought it would be fun to document.
Here are some of the color patterns that are possible in juvenile Red Rock Crabs (below). [As they grow, after several molts, the crabs will become uniform red.]
Figure from Krause-Nehring, J., J.M. Starck, and A.R. Palmer. 2010. Juvenile colour polymorphism in the red rock crab, Cancer productus: patterns, causes, and possible adaptive significance. Zoology 113: 131-139.
This is a fascinating story. The basic hypothesis that's been proposed for why there are so many juvenile color patterns involves natural selection imposed by visual predators (e.g., fish) that form search images. As rare color patterns evolve, they avoid detection by visual predators and are successful. Eventually predators might catch on to the new color pattern if it becomes common, but new patterns that arise will again enjoy an advantage. Over long periods of time, more and more color patterns may accumulate through these processes.
If you encounter juvenile Red Rock Crabs when you're tidepooling, it's fun to see how many different color patterns you can find!