Looking better all of the time!!!! Here are my suggestions:
Add drybrushed metallic highlights to the whole tank, and especially to the tracks, where they would have had wear patterns.
Add black shadows to the tracks like you have done to the engine vents.
Add a wash of raw or burnt umber to the tank, it looks good like rust, mud, etc...
Add rust, grease, oil, grime to the road wheels.
Keep going! Its looks awesome!!!
Scott