The trichromatic theory explains how humans perceive color through three types of cone cells in the retina. Each type of cone is sensitive to different wavelengths of light: short (blue), medium (green), and long (red). When light hits these cones, they send signals to the brain, which combines the information to create the perception of various colors. This theory was first proposed by Thomas Young and later expanded by Hermann von Helmholtz. It helps to explain why we can see a wide range of colors by mixing the three primary colors of light. The theory is fundamental to understanding color vision and has applications in fields like photography and television.