How light or dark a colour is, on a scale from black to white, independent of hue.
In the HSV colour model, value is the maximum of the red, green and blue channels — distinct from HSL lightness, which averages the maximum and minimum channel.
HSV value and HSL lightness are both called 'brightness' informally but are calculated differently and can disagree slightly for the same colour.
A pure, fully saturated blue and a pale sky blue can have similar HSV value but very different HSL lightness.
Value contrast (not hue) is usually what makes text readable against a background — see the contrast checker.
Value has two unrelated definitions. HSV value is the largest of the red, green and blue channels expressed as a percentage, as in the transform Alvy Ray Smith published in 1978. Munsell value is a visual lightness scale from 0 for black to 10 for white, built from observers' judgements. A Munsell value of 5 is a mid-grey. An HSV value of 50% is whatever colour has its brightest channel at half.
| Colour | HSV value | HSL lightness | CIELAB L* |
|---|---|---|---|
| Yellow #ffff00 | 100% | 50% | 97.1 |
| Blue #0000ff | 100% | 50% | 32.3 |
| Sky Blue #87ceeb | 92% | 73% | 79.2 |
| Navy Blue #000080 | 50% | 25% | 13.0 |
Yellow and Blue both have an HSV value of 100% and an HSL lightness of 50%, but to the eye they are nowhere near each other: L* 97.1 against 32.3. HSV value only reports the largest of the three RGB channels, so it cannot tell a bright channel the eye is sensitive to from one it is not. Navy blue's value of 50% corresponds to an L* of only 13.0.
How this was calculated: HSV value is max(R, G, B) as a percentage; HSL lightness is the mean of the largest and smallest channel; L* is computed through XYZ under D65.
A painter squints at a subject, or converts a reference photo to greyscale, to check 'values'.
A greyscale conversion that takes the HSV value channel makes a saturated blue as light as a saturated yellow, and the tonal plan of the picture is lost. A luminance-based conversion keeps it.