Sunlight pouring through the atmosphere is richer in violet than in blue once Rayleigh scattering has done its work, yet the open sky registers as blue to a typical observer. In the upper air, gas molecules scatter short wavelengths more strongly, so violet photons are flung across the sky more efficiently than green, yellow or red.
The puzzle lies in the interface between that radiation field and the human retina. Short-wavelength cones, medium-wavelength cones and long-wavelength cones do not sample the spectrum evenly; their sensitivity curves and the brain's opponent-process coding act as a kind of perceptual filter. Ultraviolet and much of the violet band are partly absorbed or blocked before reaching the photoreceptors, and the residual signal from the sky produces a neural response whose effective color coordinate falls in the blue region of standard color spaces.
In physical terms, the atmosphere tends to maximize scattering cross-section at the violet end of the visible spectrum, while the visual system's equivalent of an entropy reduction step compresses that rich spectral information into a simpler perceptual category. The result is a stable, efficient representation that labels a predominantly violet-scattered dome as blue, turning a complex radiative transfer problem into a single, familiar color experience.
