Does anyone use dye for their plastic?

There are a lot of dyes used in plastics. If a plastic article is colored and still transparent it is normally a dye. In polymer science we normally define a dye as something soluble in the plastic matrix. A pigment can be highly dispersed in the plastic but remains in small domains in the resin. In most cases the dyes are meter mix into an extruder either as a color concentrate or powder. If a dye is soluble in most commercial plastics then often it is not highly soluble in water. Even for wool the dye vats are normally heated. To increase the solubility you must add another solvent. At this point you have a VOC problem and waste water problem.

The only industry that routinely dyes plastic in solvents that I have encountered is the ophthalmic industry. Lenses are tinted different colors. A lot of complex dye printing has gone to dye sublimation techniques. The dyes are printed onto rolls dispersed in a resin. The printed carrier is then heated in contact with the plastic film or fabric and the dye sublimes into the film or fabric. The pattern can be made at printing for large designs or individual colors element like in Fargo ID printers. Fargo and other ID printers for example print color pictures on millions and millions of ID cards per year, therefore yes a lot of people dye plastic and chance are you have one in your wallet.

I have considerable experience using dye, post moulding for plastics. Most of this was done for actual knife handles. The dye does not penetrate far into the part, so removing some material by a process like buffing yields a mix of the dye color and the substrate color. This is frequently used to create a simulated horn or bone appearance. Care and control are required to produce a consistent result. Nylon is also quite dyeable post moulding and I have used that to give different colors to parts moulded together in a family mould.