Use acetone, then leather dye. You will need to scrub the acetone into the parts you want red with a toothbrush or hand scrubber, once you have done this 2-3 times the leather should be prepped enough for the red leather dye to take hold. You will need to add several coats to hide any of the black. I used this same method to make black boots white for my Stormtrooper.
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