For ages nature has provided man with colour in his surroundings; and over the last 5,000 years or so man learned to transfer some of her colours to cloth, paper, wood, leather, soaps etc
The vegetable dye known to have been in use the longest is indigo. An indigo-dyed garment dating from about 3000 B.C. was found in the ancient Eygyptian city of Thebes. A process called mordanting-treating the material to be dyed with other substances that serve to fix the coulour-was discovered, probably in India, around 2,000 B.C.
General Information about Dyeing

First of all you will need:

1. A scales to weigh the plants parts and the material to be dyed etc. A large postal scales or kitchen scales would be suitable.
2. A stainless steel pot large enough to comfortably hold 4 to 5 gallons of liquid and materials to be dyed.
3. Measuring cups for liquids.
4. A cooking thermometer
5. A stick or long handled wooden spoon for stirring.
6. Plastic measuring spoons.
7. Rain water.
8. Large Buckets for rinsing.

Here are some suitable mordants, you can find these at the pharmacy or your local supermarket.
Acetic acid (vinegar would do)
Alum (potassium aluminum sulphate)
Ammonia
Blue vitrol
Caustic soda
Chrome
Copperas or green vitrol (ferrous sulfate)
Cream  of tartar, or potassium acid tartrate (potassium bitartrate)
Lime (calcium hydroxide)
Tannic acid
Tartaric acid
Tin (stannous chloride)

Raw animal fibers such as wool and silk have a greasy coating that must be removed through repeated washing with a mild soap and water. Vegetable fibers do not need washing. In mordanting the clean prepared material is simmered (wool) or boiled (cotton, linen) or soaked in hot water (silk) in which the mordants have been dissolved. After the prescribed time the material is removed, rinsed and allowed to dry. The dye bath is then prepared by soaking the chopped plant material in water overnight and then boiling until sufficiant colour is extracted. The plant material is then strained out and water added to make 4 to 4 1/2 gal. of lukewarm dye bath, to which a pound (dry weight) of wet yarn or fabric is added. Wool, cotton and linen are usually simmered in the dye bath; for silk the temperature must be kept at 160 F or less. After dyeing and stirring as long as nessesary to get the material dyed through one series of rinses, each a little cooler than the previous one, until the rinse water remains clear. After drying the dyed material is ready for use.

Typical mordanting instructions for one pound of wool (dry weight):

Heat 16 to 17 litres of soft water until it is lukewarm. Add 3oz/85g alum and 1oz/28g cream of tartar which have been first dissolved in a little hot water. Immerse wet (but not dripping wet) wool in the water; spread and stir to ensure even coverage. Heat gradually to boiling and then simmer for an hour turning the wool occasionally. When the bath is cool enough to let the wool be handled, remove the wool and squeeze (don't wring) out the excess liquid. Place loosely in a bag or towel and let dry slowly in a cool place.

Typical dyeing instructions for one pound/ 450g of wool (dry weight):

Crush or chop about 8 dry litres of leaves, soft stems or flowers, or soak about 1lb / 450g of hard materials such as bark or wood; soak overnight in enough soft water to cover. The next day, boil for 30 minutes to 2 hours, depending on how readily the colour is extracted. Strain out the plant matter and add water to make 16 to 17 liters of dye. After heating the dye bath to lukewarm, add the mordanted wool, which has first been wetted in lukewarm water. Move the wool back and forth and lift it in and boiling and simmer for 30 minutes or more. When the colour is right, rinse the wool in buckets of successively cooler water until the rinse water remains clear. Squeeze out and hang it up in a shady place to dry.

List of Plant Dyes by Colour

BLACK Barberry Black alder Blackthorn Common plum English oak Flowering ash Logwood Meadowsweet Valley oak Yellow dock BLUE Bearberry Cornflower Dogs Mercury Elecampane Hollyhock Indigo Logwood Meadowsweet Mesquite Pomegranite Privet Sweet potato Woad BROWN Alder buckthorn Bird's tongue Black alder Black birch Black gum Black oak Blackthorn Black walnut Canoe birch Cascara sagrada Catechu Cotton Dyer's camomile English walnut Ginko Heather Hemlock spruce Iceland moss Indigo bush Juniper Larch Logwood Lombardy poplar Osage-orange Pomegranite Rooibos Rose of China Sumac Sweet potato Turmeric White oak GOLD Black oak Dyer's camomile Fistic Goldenrod Lily of the valley Osage-orange Privet Smartweed Turmeric GRAY Bearberry Black alder Blackberry Bracken Butternut

GREY (Cont) Logwood Red maple Rhododendron Rose of China Rowan St Johnswort Shave grass Sumac Wax myrtle GREEN Bearberry Beard's tongue Black alder Black oak Bracken Canoe birch Coltsfoot Dog's mercury Dyer's broom Fumitory Heather Lady's mantle Larkspur Lily of the valley Lombardy poplar Meadowsweet Motherwort Nettle Onion Red maple Rose of China Scotch broom Shave grass Tansy Wax myrtle White birch ORANGE Annatto Black oak Bloodroot Calliopsis Henna Onion Sumac PURPLE Black alder Dandelion Heather Pomegranite Rose of China Tall field buttercup Yellow bedstraw RED Alkanet Alpine cranberry American ivy Annatto Barberry Black birch Blackthorn Bloodroot Calliopsis Dandelion Dye bedstraw Henna Madder Poinsetta Pokeweed Red alder Rue Safflower

RED (Cont) White birch Wild marjoram Yellow bedstraw TAN Apple Butternut Fustic Goldenrod Osage-orange Sumac Tea YELLOW Almond Alpine cranberry Apple Barberry Bearberry Beard grass Big-bud hickory Black alder Black elder Black oak Bracken Broad-leaved dock Calliopsis Chinese arborvitae Coltsfoot Cotton Dog's mercury Dyer's broom Dyer's camomile European ragwort Flowering ash Fumitory Fustic Goldenrod Hackberry Heather Indigo bush Jewelweed Lily of the valley Lombardy poplar Marigold Meadowseet Nettle Osage-organe Pomegranite Privet Red maple Rooibos Rose of China Safflower Saffron St Johnswort Sassafras Scatchbroom Shave grass Smartweed Sorrel Stickleweed Sorrel Sticklewort Sumac Sundew Sunflower Sweet potato Tansy Turmeric Virgin's bower Weld White birch White mulberry Wild crab apple Yellow bedstraw Yellow root

Ref: The Herb Book by John Lust
Stockists of herbs and plant materials