If you do use this method make sure you neutralise it by washing it in a bicarbonate of soda solution.Iron filings or wire wool dissolved in vinegar.
Thinking about it, the bag I linked may have also been treated with Neatsfoot oil and then polishDepends on the leather, some leather will take on a nice tan finish just by treating it with Neatsfoot oil and wax, no need for dye. It's always worth trying an offcut.
Sometimes you can dye leather and then it changes colour when you oil it or treat the leather with some product, so it's always worth experimenting.
My apologies, I should have said hibiscus. Comfrey gives you a khaki greenish colour. I was sure I had read that it was used for cardinal robes.Method please ?
If you don't want to go down the vinegar route an alternative is to buy some iron tables and crush them up and mix them with water.I like the evenness of vinegaroon. Make sure you neutralise it with bicarb though else you'll get a rusty knife.