Greigers used to sell the dye powder, when there was a Greigers.
After you’ve polished the agate wash it and dry it thoroughly. You
mix aniline dye powder with isopropyl alcohol, soak the agate for a
while (time varies by the agate), remove and let the agate air dry.
Place it in a warming oven, heat gently up to about 250, let cool.
The alcohol is long gone, and the dye is firmly in the microscopic
tubes. If you can find a copy there is a great paperback THE ART OF
DYEING GEMS AND GEM MATERIALS by John Neavin. He gives other recipes
which will produce even more profound color changes, and be totally
solvent proof, which the aniline/isopropyl is not.