I recently attended an eco-dyeing workshop. We had 5 pots of liquids - brown onion skins, red onion skins, eucalyptus, avocado skins and seeds, and crepe myrtle bark - which we boiled up for about 2 hours. We then added fabric (I used silk noil) and let the brews simmer for a further 45 minutes. However, as you can see from these photos, there was very little difference in what came out, apart from the way I had tied up the fabrics - and the leaves that I wrapped in 2 of the pieces (4th and 6th photos).
It was a relatively easy way of trying eco-dyeing but I don't think I'll be doing it again any time soon.