I have kept two Hill stream loaches (of the Chinese plecostomus variety) for quite a while and I have noticed many times that they will either go from a very light tan to almost a black depending on what surface they are on.
These guys look like small stingrays, and use their fins to attach themselves to the glass of the tank.
One other name that I can remember off the top of my head for these loaches is "Borneo Sucker Mouth"
If you find some I would highly recomend them for any freshwater tank. They are tank cleaners, but do like a nice dark place to hide in.
Article with pictures of them: http://www.loaches.com/species-index...-kweichowensis