what colors were available for Joseph's coat?
white, (plain wool
yellows (onion skins)
greens (various green plants)
browns (various tannins--from tree bark)
reds (various insect shells yeild red)
grey, brown, tan and black (natural woolens again)
indigo was already being used in india, and i supect blue would be available.
(purple was available, but very expensive)

many plants will make wonderful dyes, all that is needed is salt for mordant. onion skins will give shades from the palest yellow to deep amber gold. most plant dyes will give deeper colors the longer the fabric sits in the dye bath.