Far from being any kind of leatherworker, I know that when attempting to recolour shoes there is a substance that strips the shine, gloss etc off so that the new dye can penetrate the leather. I have used it several times and the leather has not bee affected. I have purchased this from the local bootmaker.

You may find once you take the shine off that the white maybe able to be washed out in warm water as I did with a blancoed sporran bag