Why is it that some wings are not painted on commercial jets? Couldn't they paint them the same color as the fuselage or would it be a waste of money?
Fuselage present a large vertical surface with greater length than height. Wings are a large horizontal surface that is shorter in the direction of travel. So...
The fuselage paint job is clearly visible on the ground, painting on the wings only if you are above or below the plane. This is probably the main reason. Simple geometry. It would be waste of money.
Sun will heat a large horizontal surface more than a large vertical surface, so making the wings white is more useful than making the fuselage white. While I am not sure that wings would be structurally weaker if heated, the heat would cause thermal expansion, which would indirectly result in increased drag by making maintaining optimal shape more difficult.
Paint has significant effect on drag, more specifically it affects when the boundary layer transitions from laminar to turbulent. In the very long fuselage the difference is probably insignificant and you can paint them as you wish, but for wings the paint should be as smooth as you can make it. While I doubt white paint is smoother than paint of any other color, keeping it simple is probably a good idea.
A number of aircraft manufacturers do paint the wings; only that the color is more neutral (like white or gray) rather than the flashy aircraft liveries as found in the fuselage and vertical tail. According to Boeing, the wings are indeed painted:
Wing skins are made of bare aluminum and are protected by an impact-resistant paint system.
Modern wings are composite and are protected by paint anyway:
Protective paint is used in certain areas to prevent corrosion, and it is used on all composites to prevent erosion and moisture ingress.
It appears that the wings of Airbus aircraft are also painted:
... operate within the Airbus UK wing production facility, painting all narrow-body wing sets.
As for painting in airline livery, it would add another coat of paint, adding weight (For example, paint in A380 weighs half a ton already). Not a lot of people is going to see the wing paint anyway.
American Airlines didn't paint many of their older aircraft, leaving them mostly bare metal, to save weight and thus fuel. However, their newer aircraft are painted because it's required to protect composites.
Regarding colors of paint, most planes are mostly white because it absorbs the least heat, doesn't fade and makes leaking fluids or corrosion more obvious. Any other colors used are typically the minimum required for branding.
In some military aircraft, like the P-51 Mustang, the wings were often not painted. This is because the rough camouflage paint made the P-51 Mustang go about 8 miles slower. I don't know if this is the same reason for commercial planes, but it was for the P-51 Mustang and other military planes.
Many commercial planes actually do have paint on the wings, but the paint is white, or light grey.