I don't know how much paint weighs, but I know it costs $70,000 to paint a 737.
Or was that a 747. Anyways, it's not cheap, and then there is a weight penalty. American actually doesn't paint their planes at all. The silver is actually the color of aluminum. However, they do a lot of polishing of that aluminum. That requires chemical solvents and elbow grease from what I understand. (Probably has the same pricetag as painting.) AA claims the weight savings is immense, but I'm not so sure since no other airline does it. You'd think that they would all follow if the savings were so good.
Some people have claimed there are advantages of having different colors. Like, white reflects more heat in the summer. (What people don't realize is that black while absorbing heat also releases heat much faster than things painted white. That's why things like computer chips are black.) I have heard some people complain about US Airways' F100s getting too hot now that they have dark paint. I've also heard that ATCs at different airports complained about UA's new scheme because they blend in with the horizon if the ground is not flat and are very hard to see. A maintenance truck crashed into a jet at IAD once because he simply couldn't see it at night.
Why don't they paint the wings? Why add extra weight that nobody will see? Also, don't the wings have to be as smooth as possible to generate lift efficiently? I have a feeling that little bumps from inconsistencies in the paint could wreck that, but you'd have to ask an aeronautical engineer to be sure.