Marbled crayfish (a.k.a. Marmorkrebs) have a lot of individual differences due to variation in how they develop (Vogt G, Huber M, Thiemann M, van den Boogaart G, Schmitz OJ, Schubart CD. 2008. Production of different phenotypes from the same genotype in the same environment by developmental variation. The Journal of Experimental Biology 211
(4): 510-523. Production of different phenotypes from the same genotype in the same environment by developmental variation -- Vogt et al. 211 (4): 510 -- Journal of Experimental Biology
). That's the most likely cause of the colour differences.
The second most likely cause is some minor environmental difference that isn't noticeable. Just because animals are in the same tank doesn't mean that they're necessarily picking up the same food. Crayfish tend to set up dominance hierarchies, and the dominant individual may be getting more and better food.
likely explanation for the colour differences is hybridization. There's no scientific evidence that marbled crayfish hybridize with other crayfish species. That's not to say it's impossible
, because some crayfish do hybridize (for example, Orconectes
; Perry WL, Feder JL, Lodge DM. 2001. Implications of hybridization between introduced and resident Orconectes
crayfishes. Conservation Biology 15
: 1656-1666. Implications of Hybridization between Introduced and Resident Orconectes Crayfishes - Perry - 2002 - Conservation Biology - Wiley Online Library