Hi,
i never saw any changes of the colour with and without tankmates. I own my 13 Pleuros since they are juvies and they stay till 6 cm without tankmates. after they reaches the 6cm they got feeders, so not really tankmades. Later they share the tank with an Aquarunichthys torosus which is a very good tankmate, because he stay all the time on the lowest water region and under his big root. I also noticed no changes in colour during the feeding, all given food (bloodworms, earthworms, sticks, feeders, frozen fish , shrimps, and so on.) didnt have an effect on the colour. Even the "pimped" froozen fish which i prepared with cyclopze eeyze didnt have an effect.
The most effect i recognize was as they reached the 20cm but also the smaller ones shows those colours think it conneceted with the age of my pleuros and the ammount of light which is reflected from them.
Ok after waterchange and cleaning the glas they looked a little bit stressed, but this is only a short time effect.