I should elaborate more on the technique;
I am pretty sure I thinned the leather dye with alcohol to mute the color for a better match
You use a brush the same size you would use to make very fine cresting lines. Make a fine line on the outer edge of the pattern about an inch long, the dye dries quickly so you don't want to do too much at a time. Next, quickly use a wider 1/4" dry brush to blot the line you made. You blot from the outside edge of the pattern inward. If you look at my picture you can see how the line is distinct on the outside edge of the pattern and blurs as well as fades out as it goes to the center of the pattern.
Dry brushing is an easy technique to master although I do recommend you practice on some scrap skin before you tackle the real thing.
Here is another dye matched skin. The skin on the other limb was really vibrant and so it took more color to match.