UPDATE: If you happen to be in Northern Virginia, USA... these painted and weathered tanks are for sale at Huzzah Hobbies www.huzzahhobbies.com/ in Ashburn, VA. (Price is less than the cost of buying new and unpainted.)
I've been experimenting with weathering techniques and wanted to share the following. These are Battlefront resin Sherman's that originally came in the 2nd edition starter set.
These were primed gray and painted a base coat of Vallejo 887 Brown Violet. Tracks and wheels were base coated black, and there is a variety of colors on the crates and tarps. These were painted quickly to what I consider a "wargame standard." Next, I applied GW gloss varnish and a black magic wash. Once I was satisfied, I began to weather the tanks to look like they've spent days in the bocage.
For this weathering experiment, I used pastels (ground to a powder) mixed with Vallejo matte varnish. I use a file to turn the pastels into powder, and a case for bead crafts from Michaels works great for storing the the various colors.
I mixed a few varieties of browns with the matte varnish in a FoW blister and applied the "mud" using a disposable acid brush. There is also some very dark gray mixed for exhaust dirt on the engine cover.
After the weathering was applied, I sprayed them with Testors Dullcoat.
Below are a few photos of the results. I think they still need some work, and the black from the engine exhaust is a little overwhelming. But, at 15 mm scale, I think these would look okay on the game table.