KMn04- potassium permanganate is a very strong chemical oxidant that water treatement plants use for taste/odor in surface waters - be very carefull with it as when it is mixed with other certain chemicals...bad things can happen.
It's about the same color as Poke "salad" berries - a nice purple, but darker. The berries also make a good stain - ask any 'possum
and the berries are safer, too.
I second the walnut hulls, rotten black of course.- It's what I dye my steel traps with (ie.coil springs)