Pleco's for the most part are indeed chameleon's as Ashley states. Though they do it for a purposeful reason. As a nocturnal fish, they'd rather be hidden during daylight hours. They have adapted the color change in the wild and when in the home aquarium, will change their color the to background in order to hide.
I was amazed at this at first but to really get an idea of how spectactular this really is was when I moved mine from my pink graveled 30
gal to my blue/green gravel 75
gal. When laying on the substrate under some plant roots, he is a light blue/silver color. When he is laying on the driftwood (either right side up or upside down) he takes on the color of the wood and is barely noticeable.
Truly amazing.