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The retina contains three types of cone receptors, each sensitive to different wavelengths of light: red, blue, and green cones. These cones respond to different ranges of wavelengths, and color perception arises from the relative responses of all three cone types working together. This foundation of the trichromatic theory explains how humans perceive the full spectrum of colors.

Most color-blind individuals can see some colors but not others, depending on which cone type is defective or absent. For example, if red cones are not functioning correctly, a person may struggle to distinguish between red and green. This selective color loss directly supports the trichromatic theory by demonstrating that specific cone types are responsible for perceiving particular color ranges.

The opponent-process theory proposes that visual system cells respond to color pairs: red-green and blue-yellow. A cell excited by one color is inhibited by its paired color. This theory explains why certain color combinations cannot be perceived together, such as reddish green, and accounts for afterimages that appear when staring at a color then looking away.

Afterimages occur due to fatigue in the opponent-process color system. After staring at a red object, the red-green visual system becomes fatigued and temporarily rebounds when you look away, causing a green afterimage to appear. Similarly, staring at yellow produces a blue afterimage. This rebound effect demonstrates how opponent-process cells inhibit their paired colors.

Both theories are correct because the eye and brain use both methods to code colors. The three types of cone receptors in the retina connect to retinal ganglion cells, which convert the three-color code into the opponent-process code. For example, green cones may inhibit a ganglion cell while red cones excite it, creating a dual coding system for efficient color interpretation.

In color-matching experiments, participants adjust the intensities of three wavelengths—usually red, green, and blue—to match a target color. The fact that any color can be matched by combining these three colors supports the trichromatic theory. For instance, yellow perception is achieved by activating red and green cones together, demonstrating the three-cone basis of color vision.

Retinal ganglion cells receive input from the three types of cone receptors and convert the three-color code into the opponent-process code used by the brain. A ganglion cell might be excited by red cones while inhibited by green cones, or excited by blue while inhibited by yellow. This conversion allows the visual system to process and interpret complex color information efficiently.