I find this whole theme fascinating and am intending to get back into using natural dyes and making my own paints. Eco prints are fabulous organic things and can be made on fabric and paper. Here is an article that I found that explains 'scouring', the textile term for cleaning fibres prior to dyeing.
Below are a couple of test pieces, the silver grey is from Yarrow on silk. The reddish colour is from Mahonia berries with a slat mordant, on silk (right) and wool (left).