Posterize
This Studio Ultimate effect lets you control the number of colors used to render each frame of the clip, all the way from the full original palette down to two colors (black and white) as you drag the Amount slider from left to right. Regions of similar color are coalesced into larger flat areas as the palette shrinks.
L to R: Effect off, “Posterize”, “Max posterize”. In “Max posterize”, each pixel of the original image is converted either to black or to white.