Clean Helix Sky

Old English clǣne, pure, free of dirt — used as a color modifier since the medieval period for hues that read as crisp and uncontaminated by other pigments. Clean white, clean blue: moderate saturation combined with optical clarity. Sits at the crisp-bucket center alongside clear and true.

Greek ἕλιξ, spiral-or-coil. As a color modifier, helix implies a planetary-nebula-and-spiraling-coil quality, the visual register of Helix-Nebula-and-Eye-of-God hand-planetary-nebula-and-spiraling-coil Helix-Nebula-and-Eye-of-God-and-NGC-7293 helix-and-planetary-nebula-and-spiraling-coil surfaces under Helix-Nebula-and-Eye-of-God-and-NGC-7293 Aquarius-and-Hubble-deep-field planetary-nebula-light. Sits at the modifier-and-cosmic end of the grid, parallel to nebula and corona in usage.

The blue of a clear sky at noon — produced by Rayleigh scattering, the preferential dispersion of shorter wavelengths through atmospheric molecules. Air itself is colorless; the color we see is sunlight scattered toward our eyes by every cubic kilometer above. The reference shade is mid-latitude noon under a high pressure system: a clean, slightly green-shifted blue with the luminous depth of light scattered across an entire hemisphere of air.