If you catch bass before the spawn they are usually in 15'-3' of water during pre-spawn and they are very dark from spending the winter and scrouraging for food in the deep 16'-+ parts of the lake. When you catch them during the spawn they are in 6'-2' of water and are much lighter in color already from just being up in the light spectrum in shallower water which would explain the lighter colors. All fish are this way, and I know some of it has to do with stress I think that pre-environment before the purchase and lighting have a lot more to do with how light or dark a fish's coloration is.