I'll second the walnuts. I made my first batch of walnut dye around five years ago and after a bit of experimentation I tucked the store bought dye away and haven't touched it since. The walnut dye is period correct and can give a hue range that's almost impossible to believe, you can get a light tan to an almost black simply by varying the soak time and dilutution level.
The recipe is simple, fill a bucket with fallen walnuts (its better if they're starting to turn brown), cover with water, top loosely with a lid and let set for a minimum of two weeks or untill the husks become squishy and easy to remove. At this point you can either leave the husks on or remove them, I prefer to husk the nuts as it exposes more surface area, place husks and nuts in a cloth sack (an old pillow case works well) place in a pot, top off with the soaking liquid and get the pot boiling. Reduce to a simmer for 3-4 hours, you'll want to reduce the liquid a bit, but try not to reduce much past the 50% mark, top off with water as needed. Let cool and decant (if you kept everything in the cloth bag then straining shouldn't be needed) and add one cup of rubbing alcohol per two quarts dye to stave off mold.
Check color intensity by submerging a piece of scrap leather for 30 minutes, let it fully dry and oil it well, if it comes out much lighter than a rich chocolate brown you can add some steel wool to the dye to darken it up, the amount of steel wool depends on the quantity of dye, I use 1/4 pad per 5 gallons. The color intensity can be altered by varying the soak time, by diluting with water or by brushing it on (I use foam brushes) instead of submerging. Some people suggest cooking out the alcohol before using, but I've yet to find that a necessary step.