In this 1924 poster for the London Underground, Walter E. Spradbery limited his palette to just four colors: white, cyan, red, and green.
The resulting poster has a several advantages over an image with a full range of tones and colors. It's cheaper and easier to print, and the simplicity gives it more visual impact from a distance.
Here's a detail of the original painting in opaque water-based colors. Where possible he eliminates outlines around small shapes and lets them blend together.
It's a challenge to design a picture with just four colors. One naturally wants to subdivide, gradate, soften, and outline. But a painting gets its power from the grouping of tones, and this regime forces the artist to group, generalize, and simplify.