Shibori Technique - I bet you could use it for yarn or roving really easily too!
alexfultondesign: Visiting: The School, Sydney - Shibori Class by Joanna Fowles
dye / over-dye
Eco-dyeing - you can get beautiful colours using for example purple cabbage - lots of ideas on this blog
reference for dye colors for organic materials and what materials work well
Eco printed silk scarf with sumac leaves, onion skin dye, modified with iron. DianeGamm on Etsy
by Jude Hill....contrast colour circle stitching on denim patches
This is a 10 stitch Blanket which spirals till you run out of yarn. Now this is a great way to use up small bits of yarn..