Plain Stir 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 styrian, to-move-or-agitate. As a color modifier, stir implies a slow-moved-and-rippled-and-agitated quality, the visual register of cauldron-and-tea-cup-stir hand-slow-moved-and-rippled-and-agitated cauldron-and-tea-cup-and-pot-au-feu stirred-and-slow-moved-and-rippled-and-agitated surfaces under cauldron-and-tea-cup-and-pot-au-feu kitchen-hearth-and-witch's-fire-and-parlor-tea-table simmer-and-eddy-light. Sits at the modifier-and-mood end of the grid, parallel to swirl and eddy 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.