Maybe just give them a blackwash , it should flow easily off the tip of the brush to cover the rubber parts . I think it would give the wheels a dusty rubber look ( the base color should show through ) , which should suit your theatre of operations . If in doubt of the result , try it on the under side of your tank first as a test . You do paint the under side , don't you ? I always try washes and effects paints on the belly of my tanks before taking the plunge