This yahoo group is a good resource, sometimes gets a little tumultuous but you can go search thru the group for background help
groups.yahoo.com/group/NaturalDyes/
This yahoo group is a spin off of the above group, a lot of focus/discussion on environmental concerns
groups.yahoo.com/group/SustainableNaturalDyePractice
This site always has great info in it
and of course Ravelry has a couple of active dye groups, lots of resources...