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The Young-Helmholtz theory, also known as the trichromatic theory, is fundamental in understanding how we perceive colors through our eyes. This theory was developed by Thomas Young and Hermann von Helmholtz in the 19th century. It suggests that our ability to perceive colors is based on the presence of three types of color receptors, or cones, located in the retina. Each type of cone is sensitive to one of three different wavelengths corresponding to red, green, and blue light.

• Red-sensitive cones primarily detect longer wavelengths.
• Green-sensitive cones respond to medium wavelengths.
• Blue-sensitive cones are most responsive to shorter wavelengths.

Our brains interpret a broad spectrum of colors by combining the varying levels of stimulation from these three types of cones. For example, when both red and green cones are stimulated equally, we perceive the color yellow. This theory adequately explains color vision at the retinal level but does not account for more complex visual phenomena, such as afterimages.

The Hering theory, known as the opponent-process theory, offers a different perspective on color vision and was proposed by Ewald Hering in the late 19th century. According to this theory, our color perception depends on a system of opposing pairs of colors processed in the nervous system. These pairs are red-green, blue-yellow, and black-white. This means that certain colors are directly opposed to each other, and the activation of one in a pair can suppress the perception of the other. For instance:

• Activating red will inhibit green perception and vice versa, creating visual conflict or suppression.
• Similarly, blue will oppose yellow, and black will oppose white.

The opponent-process theory explains complex visual phenomena such as color blindness, where individuals cannot perceive certain pairs of colors. It also accounts for afterimages, which occur when continued exposure to one color results in seeing its opposing color after looking away.

The trichromatic theory emphasizes the initial stage of color vision, where three types of receptors in the retina, sensitive to red, green, and blue light, govern how we see the world. These cones work independently or together to capture light of different wavelengths, allowing us to perceive a wide array of colors.

• Red cones capture longer wavelength light, essential for warm colors.
• Green cones are sensitive to medium wavelength light.
• Blue cones measure shorter wavelengths.

This theory has been supported by various experiments showing that any color within the visible spectrum can be reproduced by appropriately mixing these three primary colors. It serves as the basis for understanding color reproduction in TVs and computer screens. However, the trichromatic theory does not explain how color vision operates when perceiving contrasting colors or why we see afterimages of their complementary colors.

The opponent-process theory extends our understanding of color vision beyond the retinal cone level to neuronal processing in the brain. This theory posits that color perception is organized into opposite pairs that evoke mutual exclusion, meaning that activation of one process (e.g., red) inhibits its opponent color (green). Each pair:

• Red opposes green.
• Blue opposes yellow.
• Black opposes white.

This theory is crucial for explaining phenomena like color contrast, where adjacent colors affect perception, and color constancy, where colors are perceived as the same under varied lighting. Additionally, it explains afterimages, which occur when our eyes become fatigued by a dominant hue and we perceive its complementary color instead. The opponent-process theory complements the trichromatic theory by providing a full picture of how our visual system perceives and processes color.