fish in stores are scared and usually underfed and when taken home and feel comfy in their new suroundings they come more relaxed and show their real colors,..
when stressed they have more muted tones to be less noticed,..
Another explanation could be in the substrate. If you put a pleco in a tank with a light colored substrate its colors will be dull to try to blend in. If there put with a dark substrate there colors darken up showing you there true colors.