Use brown, ochre, then white. The pastel chalk is trapped by the paper, creating subtle tone variations. The final touch is to use a shoe-brush in parallel to panel lines, in order to "harmonize" the finish. The advantage of this method is that the sandpaper has already got a to-scale aspect of concrete. Besides, you don't need paint and won't ruin any brush. You could also use an airbrush, but I found that changing between 3 or 4 colours was rather tedious.
This method works GREAT, and I think it gives better results than commercial pre-printed runways