Thanks for your comments
I painted the vehicles in dark yellow enamels first (Humberol 93 & white) allowed to dry for 24 hrs and then sprayed hairspray over the entire surface.
For the Stug I then brushed on white acrylic in a random stippled pattern and after drying I then used a course hard brush dipped in alcohol to partially remove some of the white and fade the paint.
For the MkIV I lightly airbrushed the white acrylic on and then followed the same method as above
Both vehicles were then dirtied up with oils and pigments