Value
How light or dark something is, with the hue stripped out. Value is the property that carries structure, and it is the one that survives every kind of reproduction, degradation and colour blindness.
Where the word comes from
Value is Munsell's vertical axis, running from black to white through the greys, but the working idea is much older than the word. Renaissance and Baroque workshops built pictures in monochrome first — a grisaille or dead-colour underpainting that fixed the light and shade before any colour was laid on — precisely because they had learned that a picture right in value is nearly finished and a picture wrong in value cannot be rescued with colour.
In the studio
The traditional way to see value is to stop seeing hue: squint until the picture reduces to a few patches of light and dark, and the composition either holds or falls apart. Caravaggio and Zurbarán organised whole canvases as two or three value zones with almost nothing in between. Rembrandt worked with a longer scale but kept the same discipline, spending his lights carefully — the brightest white in a Rembrandt is usually a very small area, which is why it reads as bright at all.
Why it matters
Because value is where legibility lives. Two colours of different hue but the same lightness sit on a canvas or a screen with no boundary between them; the edge dissolves, and for a viewer with red-green colour blindness it may never have existed. Every accessibility contrast requirement is a value requirement wearing a numeric disguise — the WCAG calculation takes relative luminance and ignores hue entirely, which is the same test as squinting at a Rembrandt.
Today
Photography formalised it further with exposure zones, and film cinematography inherited the whole apparatus: expose for the highlights, protect the shadows, decide in advance how many stops of range the story needs. In interfaces the test survives in the cheapest possible form. Turn off the colour. If the hierarchy is still readable, the design is built on value. If it collapses, colour was doing work it should not have been asked to do.
Pigments used: Lead white · Bone black · Umber
Essays: Painting with light · Why old paintings look brown · How colour became a science
See also: Rembrandt van Rijn · Caravaggio · Francisco de Zurbarán