Type "Dyeing Feathers" or "Kool Aid Dyeing" and you'll find lots of information on dyeing feathers, fur, etc. using Kool Aid, Rit, Veniards. Here's a few:

http://www.flyanglersonline.com/flyt...wavedyeing.php
http://www.ronnlucassr.com/docs/Dyin...hers%20101.pdf
http://www.washingtonflyfishing.com/...ad.php?t=46906

Two tips - if you want it bright, start with white
to get black, dye it dark brown first

Regards,
Scott

ps - A.K. Best has a great book on dyeing/bleaching materials, mostly using Rit; worth a look