Everybody has their own recipe for concrete. Here's mine:
First paint the surface with cheap hobby acrylic -- some shade between middle-gray and black. The darker, the "older" the look. Don't make it too uniform--I agree that old concrete always has variability in color.
Second, lightly dust with Rustolium Desert Bisque. This is a textured paint--don't over do it. Also, this stuff (like many spray paints) will soften foam. This can be bad or good. I sometimes do it on purpose to get an old, eroded look.
