Mustang exhaust stains

I got in some airbrushing time this afternoon, and amongst other things I had a crack at the exhaust stains on the Mustang.  In the daylight I was happy with them, but under artificial light this evening I think the exhaust stains are too brownish - I think I need to add more black and make them more pronounced as would be expected on such a weathered, war-weary plane.

Here's my reference. I've gone a bit more than this, because I want my plane to look more beaten up and to have seen more service.

Yes, I know this is a later version of the Mustang than mine. Source.

What do you think? Add black, or leave it alone?

Dave