I'm now back working on the Chieftain and have started adding the pigments. I've used a shed load of these plus thinners, pigment fixer, wet effects fluid and a black wash to get the various tones similar to my reference pictures. I was looking for a wet outer rim to some of the wheels and a dryer inner area. By combining the various mediums I think I've got pretty close to the real thing. The upper surfaces have had a more restrained application of pigments as I don't want to totally cover the main colours.
The pictures show that the chieftains are absolutely covered in mud, even the turrets but I'm going for somewhere in between. I've now started adding some oil leaks to a few of the wheel hubs and fuel staining to the engine deck. And I've also painted up a few laser cut leaves to add to the deck and other areas were they might collect.