Human color vision is a remarkable biological system built on three types of photoreceptor cone cells in the retina, each sensitive to different wavelengths of light. This system, called trichromacy, uses short-wavelength (S) cones peaking around 420 nanometers for blue perception, medium-wavelength (M) cones peaking around 530 nanometers for green, and long-wavelength (L) cones peaking around 560 nanometers for red. The brain interprets the ratio of signals from these three cone types to produce the full spectrum of color we experience. When one or more cone types are absent or dysfunctional, the result is color vision deficiency, commonly called color blindness.

Color vision deficiency is primarily a genetic condition carried on the X chromosome, which explains why it affects approximately 8% of males but only about 0.5% of females. Since males have only one X chromosome, a single defective gene causes the condition, while females need both X chromosomes to carry the defect. The most common forms are protanopia and protanomaly (affecting L-cones, reducing red sensitivity) and deuteranopia and deuteranomaly (affecting M-cones, reducing green sensitivity). Together these red-green deficiencies account for about 99% of all color blindness cases. Tritanopia, affecting S-cones and reducing blue-yellow discrimination, is far rarer at roughly 1 in 10,000 people. Achromatopsia, or complete color blindness where no functional cones exist, is extremely rare at about 1 in 30,000.

Simulating these conditions computationally involves transforming the color values of each pixel through matrices that model how the reduced cone response alters perceived color. For protanopia simulation, the transformation eliminates the L-cone contribution and redistributes that information across the remaining channels. This causes reds to appear more like dark browns or greens, and red-green distinctions collapse. Deuteranopia simulation similarly removes M-cone contribution, producing a different but related confusion pattern. Understanding these transformations is crucial for designing accessible interfaces because what appears as a clear red-versus-green distinction to someone with normal vision may be completely invisible to someone with protanopia or deuteranopia. Effective accessible design uses not just color but also patterns, labels, icons, and sufficient luminance contrast to convey information.