in GW there are many kinds of dyes that drop randomly by killing monsters.
the specified location of where to kill and what to kill is posted as follows:
The catacombs: kill crypt fiends in the mission where you have to kil 5 of them, most common drops are yellow dye red dye and even black dye!!
must be necro to do this
if you aren't a necro then kill gargolyes but the chances of getting dyes are lowered by alot...
if you can't find any then go into the forest area outside of fort ranik and kill bandits for some dyes
the bandits drop dyes more often if you are an elementalist!!
other than that you could go past the great northern wall and kill charr for some black and yellow dye.