Discharge printing is a common screen printing technique to achieve vibrant color prints on natural dark-colored shirts.
The process uses a special water-based ink that features a chemical compound activator that can deactivate the dyes used on a natural fabric, removing the color. It essentially "bleaches" the area of the shirt that it's exposed to, revealing its natural cotton color underneath, which is usually a tan or slightly off white. This exposed fiber can then be re-dyed with a new color to replace it.
Usually when we print a dark shirt, we have to use a layer of white ink under the design to act as an underbase for the colors of your design to lie on top of. Using discharge ink, we can skip the white underbase and dye the fabric directly, making for one less layer of ink, a softer print, and a lighter shirt.
There is a catch, though. Discharge ink only works on certain colored shirts. Some shirt dyes are resistant to the discharge process: colors like kelly green, royal blue, and purple. If you try to discharge these dyes, the result is a muted, lighter tint of the original color. So if you try to discharge white onto a royal blue shirt, the print will be a light shade of blue rather than the white you intend. It can create a cool vintage look, but it won't be as brilliant or bright as a normal print.
Additionally, discharge works best on 100% cotton shirts. It's less effective on synthetic polyester blends, since only the cotton fibers will have their dyes removed, which results in another faded print