Discharge ink does not cover the shirt color; it takes it away. The activator reacts with heat and moisture in the dryer and strips the dye from the printed area, while any pigment in the ink colors the fibers at the same moment. The result has almost no hand and breathes like the rest of the garment, which is why it is a favorite for soft prints on dark tees.
It only works on fabric colored with dyes that discharge, which in practice means many reactive-dyed cotton garments. Polyester keeps its color, so on a blend the printed area looks muted and heathered. Results also vary by shade and dye lot: some colors lift to a clean off-white, others leave a tint, and a few barely change.
Before committing a run, ask for a test print on the exact blank and color. Expect a faint chemical smell until the first wash. Activated ink only stays usable for a limited time, so it is mixed for each job. More in the screen printing inks guide.