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Why can people with tritanopia have difficulty distinguishing blue and yellow?

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A circular RYB color wheel divided into twelve segments, showing a progression of colors from red, orange, and yellow to green, blue, and violet. — colour The RYB color model is being illustrated on a color wheel.

People with tritanopia—a rare form of colour blindness—have difficulty distinguishing blue and yellow because they lack functioning S-cones (short-wavelength sensitive cones) in the retina.12

According to the trichromatic theory, human colour vision relies on three types of cones that are sensitive to short (blue), medium (green), and long (red) wavelengths. Because individuals with tritanopia lack the S-cones, their visual system cannot process the signals necessary to differentiate the blue-yellow spectrum. Instead, they primarily perceive colours in the red and green regions.2

This condition is distinct from red-green colour blindness (protanopia and deuteranopia), where the absence of L-cones or M-cones impairs the ability to distinguish between reds, oranges, yellows, and greens. In the context of opponent process theory, the visual system processes colour through opposing mechanisms, including a blue-yellow channel; the loss of the S-cone mechanism disrupts this specific pathway, leading to the characteristic confusion between those hues.123