The eye provides us with basic perceptions that are interpreted by the brain. Sometimes, these perceptions differ so much from reality that we understand that our senses are fooling us.
The image above consists of only three colors: a greenish blue (RGB 0, 255, 150), dark orange (RGB 255, 150, 0), and bright pink (RGB 255, 0, 255). When the greenish blue field is overlaid with pink lines, the blue color predominates, whereas the green color predominates when the greenish blue field is overlaid with orange lines. The figure appears to be made of four colors, rather than three.
Patches of color that are physically close to each other are interpreted by the eye as being a single color. This is the principle used for color halftone printing which overlays dots of several basic colors of different sizes to simulate a wide spectrum of colors. The technique is used extensively for cartoon illustrations.
An image of Charlie Brown in the Sunday comics page, when enlarged, reveals the pattern of dots that form the picture. The rows of tiny dots are oriented at different angles to avoid Moiré patterns.