Only on the first couple though. Check your refs first, but most pics I've seen of Jumo exhausts show them getting 'blacker' towards the rear. This would be particularly pertinent with poorer quality fuel towards the end of the war, the use of which is apparent in the number of accounts that mention problems with engine starting on the Dora.
But Dan's right; a very light dry-brushing of a 'rust' or brown tone will make a difference, or a bit of pastel.