I'm going to quote myself here from a post about Tea dyeing back in January. There is a huge amount of word-of-mouth advice around about dyeing that is just plain wrong. Wrong if you want decent results that last, or historically accurate ones, that is. Hope this helps...
"The cellulose-based fibers (linen, cotton, etc.) are, unfortunately, much harder to dye well than protein-based ones (like Wool). Unless properly processed, color fastness on them can be a significant problem. Correct treatment can be a lot of work, but it beats redyeing stuff repeatedly. Most "craft" dyeing procedures ( pretty much all you'll see on the Web ) are fairly anecdotal and don't address those issues...
...I've been studyin' up on historical dyeing lately. I got excellent help from a woman I know from the Smithsonian. She is a textile expert and loaned me the following publication - The Art and Craft of Natural Dyeing: Traditional Recipes for Modern Use by J. N. Liles. Professor Lyles is both a Chemist {my mistake, he's actually a Zoologist - but you need solid chemistry for that as well - Ed.}, and a reenactor (or, as I prefer to say, an historical interpreter). While somewhat technical, this small book is very comprehensive. Despite the slightly misleading title it is probably the best publication on historical dyeing currently in print. It'll give you more answers than you have questions for."
You can get Liles' book in paperback from a well-known online retailer ( that ships orders over $25 for free ) for only $13. Then you can act like you actually know something about the subject - Ha! - like me.