Didn’t watch it all the way through, but grabbed this pic from it.
I did find this old post about exhaust.
The one thing I’ve noticed about using pastels, they are much easier to apply on a non-gloss finish, be it semi or matt.
I looked at a few contemporary colour pictures of V powered aircraft. Mostly greyish stains on the dark colours but the lighter colours tend to darken where the exhaust stains are.
Color WWII pics of exhaust stains on dark paint were almost non-existent.