Elizabeth - There is a great old book called Gemstone and Mineral
Data Book, by John Sinkankas. The copy I have is dated 1972.
There's a 14-page section on dyeing minerals, including such topics
as finding out if your mineral will accept dye, pre-dyeing cleaning,
and recipes for various colors for different stones. A lot applies
to agates/chalcedonies, which are the underlying material of drusy
quartz. Black is done with a sugar solution, red with iron nitrate
made by dissolving iron nails in nitric acid, etc., and some of the
procedures take weeks.
If you can't find a copy of the book, I'll photocopy those pages and
send them to you. However, there's so much other stuff in that book
that it's worthwhile to have your own.
Tas in snowy MA.....after a 60 degree weekend!