Richard, the holster needs to be cleaned first. There is a wax finish on them that needs to be removed before the leather will absorb the stain. Acetone will work but this is better...
https://www.fiebing.com/shop/?item=42570 Don't rub too hard with the cleaner or you will get spots that are darker than others. Apply the stain with a swab or soft cloth and let it soak in to the leather. It won't look perfect but as it dries the color will even out.