Just as Pascal already mentioned, German bunkers in WWII were usually camouflage-painted, as well as dug into the ground and further hidden with camouflage nets where this was appropriate. Patterns varied depending on the area the bunker was in (after all, a bunker doesn't move, so it could be painted to match its terrain), including fairly exotic patterns: there are several photos of a still-existing bunker in my town that has plants painted on it, for example.
As for colors, they were pretty much the same as those used on stereotypical WWII German tanks, I think: dark yellow with green and brown camouflage. For the dark yellow, Tamiya's color by that name seems a good match (which I say based on examining a few square meters of original paint on a bunker just last week), though you may want to make it a bit more tan than yellow (adding in some Dark Earth might do the trick). The brown seems to have been a fairly chocolate brown (again based on some small samples of the real thing, though the ones I examined had been exposed to the weather for 60 years); starting with Tamiya red brown and adding some black will probably get you close. I haven't found any green samples yet, but then, I haven't really looked for any, either
The interior of German bunkers was flat white, but the entrance had a triangular area of dark yellow, running from the top of the doorway down; examine some of the photos at http://www.xs4all.nl/~gurth/afv/bunkers
to see what I mean.
As for rubble from a building, it would generally have the same color as the materials used to construct that building: brick red if the building is made from red bricks, etc., but remember that a collapsing building produces a LOT of dust, that will cover pretty much everything. I think you'd need to decide what color the dust would most likely be, after which you can paint (airbrush?) most of the rubble and surrounding areas with that.