The importance of having contrasting colors on the top vs. the bottom is so that you can tell if it's inverted or not. So, you can have, say straight stripes on the top with angular stripes on the bottom, just so they are different enough so you can tell the difference between upright and inverted flight. (wing and/or stab that is).

Opjose has a great example of what I mean in his post. The top is different from the bottom, enough so that you can definitely tell the difference from the ground.

CGr.