Dye solutions are made from plant materials by heating in water to extract the colours. Fibres are prepared by mordanting with a compound that enables them to absorb the dye and helps to fix it. Then they are immersed in the dye bath and gently heated to allow the dyes to penetrate. Depending on the plant and which part is used, they can create a wide range of colours. Natural indigo blues from Woad work differently. They are extracted by fermentation and applied by dipping yarns and fabrics in the dye. When lifted out, they turn blue on contact with air.
Below are some examples of the colours that can be produced depending on the part of the plant used and the mordant.
Achillea hybrids/Yarrow: whole plant - pale yellow
Reseda luteola/Weld: tops and stalks - bright yellow