Could I , please ,get some pointers on which color to use to simulate concrete
Assuming you're talking about WWII German bunkers or similar constructions, unless your model is to represent such a bunker as it stands today, it should probably be painted in some kind of camouflage finish to more or less match the surrounding landscape. Patterns and colors used by the German army in WWII were often similar to those used on their tanks in 1943-45, just on a bit larger scale. If you're feeling adventurous, you can even paint the bunker to resemble a civilian house, barn, or other structure, with painted-on windows, doors and brickwork, or even a fake pitched roof from wooden beams and chicken wire.
And then there's the earth mound that often got thrown up around bunkers to both hide them and protect them from nearby blasts and direct fire. This would extend from all sides except those that had doors or firing ports (probably making the whole structure two or three times as wide as the bunker alone) and covered the roof in a layer of dirt as well. Effectively, this means you don't need to paint much of the bunker walls at all.