I have read that this stuff works great but I have never used it. It is sold by the quart or gallon.
It is black tire paint sold by M.E. Miller which sells tractor tires among other things. Lots of people restore tractors so I guess they have a demand for it. It is described as a water based stain and will completely cover whitewalls.
Here is the link;
Tire Supplies - m. e. MILLER tire
Also saw this solvent based tire paint from All Tires Supply, sold by the gallon. It's a concentrate and can make up to 10 gallons lol.