Hi jeff,
Neat looking Matilda. I'd agree more dust of varied colours and maybe rub some into the paint work.
the official colours called for:
BSC No 34 Slate or New Service Colour
BSC No 28 Silver Grey
BSC No 64 Portland Stone.
The Silver Grey had more of a green hue to it but I've seen plenty of models painted with similar paints so it probably depends on what paint you used.
It appears you've used Light Stone rather than Portland Stone but again that's down to what you want.
I'd maybe vary the tarps a bit.
Grand looking Tilda either way, nice work.
Al.