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About a month ago I purchased 2 x S.Petricola's and 2 x S. Eupterus. In the LFS tanks they were nicely coloured and light in colour, especially the Eupterus.

In my tank at home they seem to get darker, especially again the Eupterus. Is this normal behavior for certain Syno's to change colour.

It's perfectly normal. You probably had a darker substrate and more hiding places than at the store, and this makes the fish go darker.

You are spot on, I do have a black substrate and I do have lots of hiding places. OK, so now I know it's not something to do with their diet.

Yes it's the change in surroundings.