You can do the same with medium brown leather dye and a fine brush like one crests arrows with.
In this case I was camouflaging the splice of two short skins. While I was at it I put the same accent on each of the "hour glasses" on the entire bow so all the skins appeared to match.
I first put a thin line on the edge of the hour glass, next I used a dry brush to blot the line away from the edge of the pattern and fade it into the center of the pattern. This is an artist's technique know as...... "dry brushing", who would have guessed?
You do need to keep a little denatured alcohol and a rag on hand to wipe off your goof-ups before the dye dries.