what corydora_freak said - i have a *common* pleco (his colourations are more unusual than the garden variety but he is yet to be officially I.D.'d but kinda looks like
pterygoplichthys pardalis - maybe a subspecies?) and his brown color is patterned by some lighter markings along his body which become very apparent when he is stressed or scared. It is a good indication if water parameters are off or if there is a bully in the tank (in my case, he IS the bully) - watching your fish's behaviour and watching your fish during various times of the day will familiarise yourself to all the different colors and markings - this happens to tetras and barbs as well as many other species. The nighttime colors is in reverse for Bosemani Rainbows - for them, they
darken in color overnight (most likely to blend in with their surroundings and not be easy prey at night). So when you first turn on the lights, they will be in excellent condition to take photos. Also, ever notice how a fish you bought in the lfs suddenly brightens and colors up after a few hours in your tank? It fascinates me.
the pic below is borrowed from planetcatfish - notice the patterns of dark and light i mentioned above:
and here is my pleco in a more comfortable pose - the slightly lighter patterns on his body will turn almost while when he is unhappy or scared:
