I have 7 Amano Shrimp in my tank, 6 of them are the standard grey colour with dark sadles. 1 of them for some unknown reason to me has changed colour and has turned a really darkish brown all over.

Hes still quite active and adventurous for a Shrimp but I don't understand why he has changed colour.

Any ideas?

I'm not sure why this happens, but it seems to have no ill-effects. I have just one Amano shrimp who came to live here with some fish from Jaspersdad when he had to close some tanks down. "Shrimpy" turned quite a dark brown over the first week or two, but now she's turned back to the normal colouration.....no idea what triggered the changes, but as a 2 year old shrimp (at least) she's still going strong despite her chameleon abilities!