I believe wings are usually painted (mostly in a grey finish), that is at least with newer aircraft types. That is done to protect the bare metal and composite parts and though it adds weight it seems to pay of in the end. Check out this thread, especially reply #18:
These are also good threads:
Every airline has its own policy on how to pain the wings, e.g. LH
paints them white on top since it better matches theirs color scheme. UA
paints them in a light grey, NW
paints the middle part where you can walk on in dark grey as can be seen well in this pic:
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Photo © Sam Chui
I started a topic on this a while ago too with a few replies:
Have fun reading all that... It is interesting though.
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