Value
How light or dark a color is, independent of hue and saturation — the axis that remains when an image is reduced to grayscale.
Example usage
“Check the palette in grayscale; if the values are distinct, the layout survives any screen.”
Editor’s note
Value does the load-bearing work contrast gets credit for. Squint until the colors die: what remains is the composition, and if nothing remains, color was carrying everything — and will drop it on the first sunlit screen. Painters check value before hue; interfaces deserve the same discipline.