For those of you who think I'm completely crazy, here is some science to back the idea:
"Let's start with what is universally true: craws do change color. In a study performed by the University of Michigan, Dr. Robert Thacker found that crawfish have red and blue photoreceptors. In lakes where blue-green light transmitted best, the craws were lighter in color. And in lakes where red light transmitted best, the craws were darker in color. There has also been clear data to support that swamp-like bottom composition results in very different coloration than rocky bottom lakes or tributaries. I'm not carrying around a spectrophotometer from lake to lake measuring the color intensity, but I do have a few traps that I'll place at different points around the lake to collect samples of crawfish, which is ultimately the most effective way to match the hatch. This is really important to do because local bait shops often import what they sell from farms, where conditions can be completely different from what you're fishing.
When they'll change color is when they're molting, or shedding their shell. This results from outgrowing the one they'd worn previously. This process has also been known to change the color entirely based on habitat. What researchers have found is that molting is initiated by a chemical that's released in the body that's been linked to the different phases of the moon. An article in Bassmaster Magazine had this to say about colors and moon phase: 3-4 days before and after a full moon throw brown and orange or olive and orange,black and red as you move further into the moon cycle, new moon nights should warrant dark brown and black, and to use black and blue as you move to a sickle moon."