I had a go with using raro and the results were brilliant! For a 50g skein of yarn I used 3 packets of raro (I wanted really bright results) and 1/2 a cup of white vinegar. I soaked the yarn in vinegar and water first, then made a dyebath with the raro, vinegar and about 2 cups cold water. then I added my skein, and put it on a low heat. Once it reaches a certain temperature, the water goes clear and the yarn sucks up all the colour.
So far it seems colourfast. Those weird ones like "blue raspberry" seem to work the best, it's put me off drinking the stuff though.
Will post pics this weekend of my raro yarn.