posterize() cuts every colour channel down to a few even steps and nothing else: each colour moves to the nearest step, so a gradient turns into flat bands. It is the plainest way to take colours away, and the bands are the point: a comic, a poster, a toon look.
Its one number is effect.uniforms.levels, steps per channel. Two steps is eight colours in all (two per channel, cubed); 32 is what a SNES could store and is close to invisible.
Click to change the steps and Space to switch it off. D switches to dither() at the same steps: the same colours available, and the only difference is the pattern that hides where each band ends.