I experimented with painting and found that using just primer worked the best.  It gave a nice flat finish and put a load of color on the neoprene with a very small amount of paint.  Use too much paint and the neoprene will start to get stiff and the paint will just flake off.  Use just a misting and it sticks and stays.

So first I did a light coat of white clean metal primer.  Using white first was important.  When I tried to get to the target color without it, I couldn't without a load of paint.  Spray from a good distance, about twice as far as normal.  It doesn't take much at all to get to about the 50% target coverage.

Then a somewhat random 25% coverage coat of rusty metal primer (reddish brown).

Then it was finished off by spraying it from the top down with some lime green paint, then some darker green fluorescent paint.

While it is still a little wet, you can randomly rub some of the areas which would see wear in the real world (if Groot was in the real world) to darken those areas.

No pictures of this, sorry.  I may do a follow up video later and add it here.