A dyer is someone who can dye cloth and thread for use by clothiers. Dyeing is a difficult job that involves chemicals and powders that cost exorbitant amounts of money for the rare colors. There are more common colors that are used by lower ranking gentry, but poor folk generally do not have dyed clothes, as the cost is just too great.
Black: Not that common, but can be hard to achieve on most fabrics. Made with oak galls, blackberry leaves and urine.
Yellow: Was made with onion skins, weld or turmeric powder. One of the more common dyes.
Orange: Made with peat and leaves. Sometimes comes out as brown and has to be ammonia treated. Quite common.
Brown: Tannins produce this color. Extremely common and easy to make. Sometimes a byproduct from other dyeing processes.
Blue: Gained mostly from woad or glastum, but could also be produced from indigo and lapis lazuli stones. Very expensive, but quite common in some parts of the world.
Green: Quite difficult to make. Made mostly from tumeric and then indigo. Lichens can produce a dull green. Most batches will turn out brown unless the mixtures are completely correct. Ammonia helps bring out the green from the brown.
Red: Some tree leaves, madder and some ground insects can produce red. The madder is likely the cheapest option. Can also be produced by overdyeing orange and letting it soak in urea. It'll produce a matte, washed out red color. Pure red would require the above methods.
Purple: Rare and expensive seeing as it requires first dyeing something red as a base and then dyeing it blue.