I used to really want a flip-top. One day at Carlisle a few years ago I started really paying attention to them, and the conclusion I came to is this: things like hood scoops and spoilers can get away with being different colors and drawing attention to themselves because they're saying "look at me! I've got performance!" (even if it isn't true :-) ). But a gas cap isn't something that you really want to draw attention to. The message there is "Look at me! I keep fumes in the tank!" So, me personally, I go with body color.