1) Get some RIT dye remover and use it first
2) Then dye with walnut hulls, logwood, strong coffee, or even Rit Dye itself - with any of the above add a cup or so of vinegar to set the dye. Let air in the sun and wind.
If you'd like to make if look more like braintan stetch it as TIGHT as possible smooth side up on a piece of board. Use an orbital sander with 220 grit - if that doesn't do it use 150 grit. Just go slow and keep the sander moving. Do this before anything else.
While the reenacting community at large considers yellow to be the mark of a greenhorn there are in fact plenty of original 19th Century NDN gear dyed in a bright yellow - in particular So Plains clothes ie. So Cheyenne, Kiowa, and Comanche - they are of course braintan rather than smooth side commercial hides.........