Plain Inn Sky

Latin planus, flat, level — used as a color modifier since the medieval period for hues that read as undecorated and direct. Plain white, plain blue: moderate saturation, no shift, no surface effect. Sits in the crisp-bucket center, with the implication of restraint rather than absence.

Old English inn, lodging / dwelling. As a color modifier, inn implies a coach-stop-and-tavern quality, the visual register of English-coaching-inn-and-Scottish-Highland hand-built stone-and-timber way-stop tavern-and-stable surfaces under candlelit-and-firelight English-and-Highland coaching-inn evening light. Sits at the modifier-and-place end of the grid, parallel to tavern and lodge 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.