EnderIO in For Newbies (A Reference for the Rest Of Us!)
It takes 6 coal powder and a slimeball to make 2 black organic dye. It takes 12 'clippings and trimmings' and a slimeball to make 2 green organic dye. But you can get 312 (average 69) per Cactus, which means you only will typically need 23 cactus at most per 2 dye, or roughly a 1:1 exchange on average.