I've got a big hide of pale green, very thick, and very soft leather. It was incredibly cheap, probably because of the disgusting color.
I made a leather sheath for my sword out of it, by just cutting two pieces a little bigger than the shape of the blade and joining them by thick black leather thongs at the sides. Seeing the still-disgusting color, I added some patterns with a black waterproof pen. It didn't help much. Additionally, the sheath is very floppy because the leather is so soft.
Silk color is fixated by heat. With a silk scarf you'll do that by ironing it when it's dried. With leather, I figured I could bake it in the oven at a low temperature. So I put the leather with the still wet color and the salt on top into the oven (100°C) on a cookie sheet.
The effect was astonishing: The leather came out harder, looked reddish-brown and has a very natural pattern as if from years of use and staining. The color is water proof and doesn't rub of as far as I can see.
The easiest way I found was waxing it.
What you do is this: You melt down candle wax. The small white candles you put under your tea pot work perfectly. Take out the wick and melt the wax down carefully in an old pot or a glassware coffee pot. Do it very carefully with little heat. Wax is inflammable. If you are the careful type or do it for the first time, boil water in a bigger pot and insert the pot with the wax into this one. This way you'll never get over 100°C.
Heat the leather in the oven at the lowest possible temperature your oven provides. Mine does 50°C to start with. Then rub the molten wax into the warm leather from the soft side. Use gloves for that. Normal household gloves did the trick for me. This also shows you that the wax I used wasn't too hot. I could have touched it with my bare hands without getting burns.
The leather soaked up the liquid wax. The smooth side still showed patches of dry leather, but putting it into the oven for another 5 minutes spread the wax evenly within the leather.
Try out different types of wax and how much you want into the leather. More wax and wax melting at a higher temperature (mind your fingers!) will harden the leather more, but to much wax makes the leather feel waxed, too (obviously). Less wax or softer wax will leave it more bendable. You can suck out some of it by heating the piece and pressing it between sheets of tissue or toilet paper. Don't use newspaper. It rubs off.
At the end of the procedure, bend the leather around the mold or into the shape you want it and let it cool for an hour or two. It'll harden into place. If it didn't work the first time, or it gets bent again over time, simply re-heat it and shape it again.
Btw. the color is not affected by the wax. But if you do the wax first, there is no way you'll get a water-soluble color into it later on.
Experiment on this. You can get anything from sword sheaths to hats and pouches. Have fun!