Logwood is a South American dye obtained from the heartwood of a tree and will give you invaluable purples, violets and blues depending on mordant and temperature. Logwood is a strong dye and like cochineal you can use 25% of the dry weight of your fibre to be dyed so for 100g you can use 25 g of dye. Mixed with equal quantities of madder you can get violets and red purples. Boiled with iron will give blacks.
Logwood is best dyed on a mordant of 22% WOF ( dry weight of fibre to be dyed) alum (potassium Aluminium Sulphate) for more information read Colours of the Earth available from this website