Plumb Umber Sky

Latin plumbum, lead — referring to the lead-weighted plumb-line of pre-modern carpentry. As a color modifier, plumb implies a clear-and-vertical-true quality where the hue carries the visual register of gravity-aligned-and-perfectly-vertical surface. Sits at the crisp-and-balanced end of the grid, parallel to level and squared in usage.

Latin umbra, shadow. As a color modifier, umber implies a shadowed-and-Umbrian-earth-pigment quality, the visual register of Caravaggio-and-Rembrandt-umber hand-shadowed-and-Umbrian-earth-pigment Caravaggio-and-Rembrandt-and-Vermeer-Dutch-Golden-Age umbered-and-shadowed-and-deep-glazed surfaces under Caravaggio-and-Rembrandt-and-Vermeer chiaroscuro-and-tenebrist-and-glazed studio-window-light. Sits at the modifier-and-mood end of the grid, parallel to shade and gloom 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.