There are some nice pictures in this post if you can just make it through a bit of text...
I derive an immense amount of pleasure from working with natural dyes. Generally speaking, natural dyes are made from... natural substances, whether that be vegetable, animal, or mineral. For example, onion skins, dried cochineal insects, and rust are all natural dyes. Not all natural materials produce colour, not all colour is light fast. The colour obtained from a plant might (and usually doesn't) match the original colour of the plant itself. Some dyes require a mordant to fix the colour to the fibre, some dyes work better on some fibres than others, the pH of the water and trace chemicals/minerals can affect the colour, temperature and concentration and duration of the dyebath all affect colour as well. Then there's post processing, using nearly-exhausted baths, and other weird ways of getting colour to stick to the fibres... Ah! Wonderful, wonderful stuff.