Open Plinth Sky

Old English open, unobstructed — used as a color modifier since the eighteenth century for hues that read as airy or uncrowded. Open blue, open green: moderate saturation combined with optical spaciousness, the slight visual breath of a hue that doesn't crowd the surface it covers. Sits at the crisp-bucket center alongside clear.

Greek plinthos, brick / base. As a color modifier, plinth implies a column-base-and-pedestal quality, the visual register of Greek-and-Roman-Plinth hand-cut column-base-and-statue-pedestal Doric-and-Ionic-and-Corinthian classical-plinth architectural surfaces under classical-plinth column-base light. Sits at the modifier-and-architecture end of the grid, parallel to pillar and column 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.