The weathering was done with tamiya earth mixed with various pigments, MIG productions rusian earth, light dust, and some other manufacturers mud, and dry mud. after this the whole mud was washed with artist oils (earth tones like the pigments) the splashes where created with pigments blowen out from the brush.
The white (light grey) stuff you asked, is revell plasto mixed with sand, deluted with revell glue. It is called premuding and it helps to create more deep texture to the mud at the weathering stage.
Experiments with AFV Club's Sturmmorser Tiger - hand painted camo, that was
then mostly covered by what has started out as crushed brick/concrete but
ended up looking like winter white-wash (or both in places):