Probably look nice as long as the stitching is neat and close to the edges of the panel, the further away from the edge the less attractive a contrast stitch to me.
Your photos above is what i call close to the edge stitching and where it looks good.
The only combo of contrast stitching I don’t like is white thread on a dark colored leather. This combo of rust and black is great. Reminds me of how true vintage black jackets often have sun faded brown stitching. Would look awesome on a PHWM.
I think that looks pretty great, personally. But contrast isn't always what you want, either, so I get it. I also like gold on black quite a bit. Black does go with a lot of colors. (And, well, that's dark seal, as you say, but it's still pretty close to black. Probably more apparently seal brown in person tho
Certainly worth careful consideration. D-pocket jackets such as Ridley & J106 have a lot of stiching on them, along with harware.I gotta admit, it’s growing on me. Not sure what it would look like on a full D Pocket cross-zip though.!
I’m with you on the olive. The rust on dark seal or black looks supurb when you just see a section, but I think stepping back to see the whole jacket it would look too busy.I also first thought of contrast, but then decided on the more discreet variant in olive and do not regret it.