Plant dyes are kind of boring. Sliver barb would be sort of interesting if it weren't bugged, but it has the same value as dimple cup and blade weed, which doesn't seem to reflect how hard it is to obtain.
I suggest an animal based dye. Tyrian purple was a very expensive dye made from a sea snail (murex, to be more precise). It could be produced by either "milking" the snails one by one, or by simply crushing the whole lot of them at once. One process is slow but preserves the snails; the other is fast but destructive.
Both routes could be open in DF. A presser could produce dye really quickly, but you might run out of snails. A milker could produce dye without killing the snail, but would take a really long time. (An animal dissector performing "Extract from dead animal" would be better if not for the "dead" part; maybe add an "Extract from live animal" task?)
There could be multiple species of dye-producing murex, maybe one found in the oceans and another kind found in the caverns. Others could be possible, e.g. freshwater varieties, or different species in different ocean biomes. Also, in the real world it took a lot of snails to make an appreciable amount of dye. Perhaps the DF snails could be larger. Perhaps giant varieties could exist. The giant cavern murex charges the weaver! The weaver doesn't notice. Okay, maybe not.
This could fit in well with the new trade system, since it potentially gives coastal settlements a valuable trade good not available inland.