I've dyed headstalls, reins, and breast collars, taking them from a light oil finish to a dark burgundy. What I use is Kelly's Leather Dye that I got at a shoemaker shop (shoe repair). I prepared the leather using alcohol then applied 2 coats of the dye ~1/2 hour apart (leather was at room temp) and let it sit overnight.
The last time I did a headstall was 2 years ago and I'm still using it several times per week with no visible color change. The headstall was over 7 or 8 years old and heavily used before I dyed it. If it starts to show wear, I'll just dye it again. IMG_0406.jpg