I just found this on another site and found it very interesting and helpful
Brew some "vinegaroon." Spray fine-grade steel wool with water and allow to rust for several days to a few weeks, rewetting the material daily. When well rusted, put the steel wool in a jar and cover with boiling vinegar. Let steep several days. Longer steeping time and rustier wool will give you the darkest color. Mix vinegaroon with a small amount of commercial leather conditioner to make it easier to apply to leather. Vinegaroon is a good choice for small objects that can be briefly submerged in the dye, such as a leather wallet.
4Brew coffee. Strong coffee can be used as a leather dye. Brew the strongest coffee you can, double the amount of grounds you would normally use, then simmer in an uncovered cooking pot to reduce it to one-third its original volume. You can also use leftover brewed coffee, reduced on the stove top, or instant coffee mixed boiled in water. Allow to cool. Mix coffee with a small amount of commercial leather conditioner before applying to leather. Coffee is a subtle pigment, but can revitalize a faded tan leather sofa by giving it a richer brown hue.
5Go natural. Red onions can be boiled down to make a shade of blue. You can also make a brown dye from the green outer skin of fresh walnuts; crush the walnut skins underfoot and boil the pigment out of them. Natural dyes will give leather a more subtle color than commercial pigments and are most suitable for doeskin or other naturally pale leathers.
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