Hiya ACHTUNG...At first, All the best wishes for 2004. And now for your question. Normally I paint the cobblestones in a flatt base coat of panzer gray, ( Humbrol matt 67). Let it dry very well, and then I add a wash of Raw Umber and Burnt Sienna oilpaint, which is dilluted for approx. 80% with white spirit. After another drying period, With oilpaint approx. 24 hrs., I dry-brush the whole thing with Revell matt 89, Humbrol matt 183, and for the highest parts, plain matt white.
It takes some practice, but it will turn out very realistic,
Good luck, John.