Concerning the Sherman, I think one thing that is overlooked is the basecoat itself. I mean that the OD green looks too uniform on most builds. This is compounded when weathering, damage, etc. is applied to features on the model without varying the OD basecoat color to match. I would apply the standard OD, and then go in with a lightened OD mixture, leaving the darkened original color on the fnges. This realistically replicates the faded paint of a vehicle in service. Do a forum search, weathering has been covered ad nauseum here in the forums, there are literally hundreds of tips and links in the armor forum, techniques, etc. to answer most of your questions.