From time to time, you will see news of a lobster being caught with some unusual colour, like orange, blue, or calico. Some even become celebrities, of sorts. And within the pet trade, brightly coloured variations of crayfish (typically bright blue) are widely prized.
What determines colour in crustaceans generally? It’s a complicated mix.
The most dramatic colour variants are caused by genetics. In crayfish, several colour morphs are due to simple recessive genes (Black and Huner 1980), of the sort you learned about in high school biology.
Marmorkrebs are genetically identical, but they are not physically identical, and this extends to their colour. The article about them in Tropical Fish Hobbyist mentions the variation that you can get in the colour. Since these differences cannot be genetic, they must be environmental.
Bowman investigated this in crayfish decades ago by placing crayfish in normal tanks, tanks painted black, and tanks painted white. Crayfish placed in black tanks had more red colouration, and those in the white tanks, more white colouration. Bowman also noted that animals that had become adapted to the bright white tanks did not darken up again after being placed into black surroundings. There are limits to how flexible the colour changes are.
Similar changes in colour have been seen with hippid sand crabs (Bauchau and Passelecq-Gérìn 1987; Wenner 1972). These crabs are diggers, and those that live in dark beaches of volcanic sand tend to have darker carapace colours, while those living in white beaches of coral sand are lighter. If they are switched to different colours of sand, they can slowly change their carapace colour for a better match.
Why might there be variation in colours from Marmorkrebs in the same tank? Even within the same tank, small crayfish are unlikely to have the same light and food. Crayfish do fight and establish dominance, so some individuals may be consistently getting the prime locations in the tank and first crack at food.
Bauchau AG, Passelecq-Gérìn E. 1987. Morphological color changes in anomuran decapods of the genus Hippa. Indo-Malayan Zoology 4(1): 135-144.
Black JB, Huner JV. 1980. Genetics of the red swamp crawfish, Procambarus clarkii (Girard): state-of-the-art. Proceedings of the World Mariculture Society 11(1-4): 535-543. http://dx.doi.org/10.1111/j.1749-7345.1980.tb00147.x
Bowman TE. 1942. Morphological color change in the crayfish. The American Naturalist 76(764): 332-336. http://www.jstor.org/stable/2457208
Wenner AM. 1972. Incremental color change in an anomuran decapod Hippa pacifica Dana. Pacific Science 26: 346-353.
Additional: See this post on why colours change when a crustacean is cooked.