What is process printing?

Also called "4-color process printing," process printing refers to the technique of printing a full spectrum of colors using halftones of only 4 ink colors layered over each other: Cyan, Magenta, Yellow, and Black (or, CMYK). These 4 transparent ink colors can be used in combination, with varying degrees of transparency, to create any color. Custom Ink primarily uses this method to print photographic images, and this process works best on light colored shirts, like white or natural, because the transparent CMYK inks tend to pick up the hue of the shirt underneath them.