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it is a coomon p***o but does his colour change quite often ??? i have 2 and mine either do what your top picture has done or turn reddish brown when they see each other
AFAIK all Loricarids are able to change colour. For instance, they will change to some more stripy/blotchy colour when "stressed" (i.e. being caught, or scared).
They can also go darker and lighter depending on the area around them.
Yep, squabbles are most likely to cause some form of stress situation, but also, maybe more importantly, it's critical to show the opponent that you don't like what they are doing, and changing colour could be a good way to do this...