Open Spar 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.

Old Norse sparri, long-pole. As a color modifier, spar implies a long-wooden-pole-for-sail quality, the visual register of Tall-Ship-and-Royal-Navy-spar hand-cut long-wooden-pole-and-spar-for-sail mast-and-yard-and-boom maritime-rigging surfaces under tall-ship-spar-and-yard maritime-rigging light. Sits at the modifier-and-nautical end of the grid, parallel to mast and boom 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.