Allen Martin doesn't have any trade secrets !! He don't know much either !!
The gun you are referring to was Wayne's Scrubby in-the-white ... It was supposed to be finished by the show but I ran out of time , so I brought her along in the nude !! She's such a show-off !!
To answer your question on what I used to highlight the wood , I didn't use anything . I never did, never will ..
If there are tool marks present (not sandpaper scratches) I will sometime leave em as parts of effect I'm seeking ..
When I have a gun stock all completed .. ready for carving or finish .. I wet the stock one with a damp rag , after it dries , I scrape with my little razor scraper ... sometimes I'll burnish but most times not .
Then I apply my yellow or golden aniline water base dye as a base coat .. then a dark brown to make the curl pop .. then onto finish ...
Darkened varnish is my deeper colors that come later in the finish .. and so on ..
Hope this answers your questions ..
Allen