1) I am using the rubber tracks... what is the best way to make them look real? Step by step please.

Dry fit first. I removed two links (I think) from the ones on the PTO Sherman.
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Good idea!

2) I am going to try using sponge high density make-up sponges to apply a brown, hull red and whatever stipple job instead of routine camoflauge. What other colors can I consider to keep it fairly dark?

Don't go overboard with improvising your camouflage. It might look unrealistic. Have you thought about ading a white chipped camo, using the sponges?
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I hadn't thought about using any white. I was going to use the box art for an example.

3) What is a good mud to use to simulate a good old Russian winter/spring thaw muddy road?

Mig makes mud in a pot.
https://www.themodellingnews.com/2013/01/sticky-situation-mig-productions-have.html
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4) How far can I go with a plain display base before it becomes a diorama?

Depends on the size, and how many figures, etc, you add.
_________________________________________________________________ Wasn't even planning on figures as to me that makes a diorama...

Thanks in advance.

I've missed this crew!

Do you want the crew in their hatches, or outside the vehicle? Do you want resin or plastic figures?
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Buttoned up... not going to do figures.

Regards,

Marc