It probably could have been either one at one time or another. Most of the pictures I've seen of them showed them in OD, but I'm sure there were some that used natural metal finish as well.
The decals used are frequently what dictate the colors of any aircraft. The decals will normally be for a particular aircraft (not just a B-26 but a specific B-26) in a particular squadron, and the squadron's colors and markings would be specific to them. Check the instructions for your kit and it shold show the colors to be used. If they don't, check around on the internet and you can probably find pictures of that particular airplane or one in the same squadron.
Edit ... I came back to edit this and saw Keyworth's post. This page:
http://b26marauder.com/b26page3.html
has some good pictures and information. This particular image:
http://b26marauder.com/b26/paste1.jpg
states, "Coloring: Topsides are standard freen and brown. Undersurfaces are light blue."