Short answer
Blacks are not truly black: they reflect a few per cent of light, and that residue has a colour — bluish, brownish, greenish — set by the dyes or pigments. Indoors the light is too dim for the eye to see such small differences; daylight is hundreds of times brighter and bluer, so the residual hues become visible, and gloss and texture differences show up in the sun as well.
A deep black fabric or paint still reflects a small fraction of light, and not equally at every wavelength. Blacks made from carbon pigment tend to be fairly neutral; blacks built from mixtures of dyes, which is most textile black, can lean blue, violet, green or brown depending on the recipe. Two blacks from different makers or dye lots therefore often differ only in the hue of that small residue. Because the residue is so small, the difference sits right at the edge of visibility and is very sensitive to viewing conditions.
A living room at night may be around 50 lux; open shade outdoors is in the tens of thousands, and direct sun higher still. Two surfaces reflecting a few per cent of 50 lux send the eye very little light, and at low luminance both lightness and hue discrimination fall off: the two blacks collapse together. Multiply the light by several hundred and the same few per cent becomes a luminance at which hue is easily seen, so a blue-black and a brown-black that were indistinguishable indoors become obviously different. The difference was always there physically; indoors the eye simply could not resolve it.
Indoor light is usually warm; daylight, especially open shade, is much richer in blue. A blue-leaning black gets more of the light it reflects best and brightens relatively; a brown-leaning black loses ground. This is metamerism acting on a very dark pair, and in daylight it adds to the light-level effect rather than replacing it. Ultraviolet adds a third, smaller factor: fibres or finishes containing brighteners, or dyes that fluoresce slightly, respond to the ultraviolet in daylight but not behind glass or under LED light.
Outdoors, a strong directional sun produces bright specular reflections that indoor diffuse light does not. On a black, the specular sheen — white light reflected from the surface before it enters the colour — can be brighter than the black's own colour. A satin weave and a matte weave, or a new and a washed garment, reflect that sheen differently, so they look different blacks even if their colour is identical. Viewed at an angle to the sun the sheen dominates; viewed away from it, the colour does. Faded blacks add yet another difference, since the dyes in a black mixture often fade at different rates.
Why: Different dye recipes whose residual hues only become visible at daylight levels.
Fix: Compare blacks outdoors in open shade before buying separates; the brightest light is the strictest test.
Why: Different materials and gloss; specular sheen and residual hue both show in direct sun.
Fix: Judge in open shade to separate colour from sheen, then in sun for the combined effect.
Why: Component dyes in the black mixture have faded at different rates.
Fix: Replace as a set, or pair the older piece with a colour rather than another black.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
Typical illuminance in open daylight shade is on the order of 10,000–25,000 lux and in direct sun higher still, while a living room after dark may be around 50 lux.
Caveat: Order-of-magnitude values from a reference table; real rooms and days vary widely.
FactModerate evidence
Perceived colourfulness depends on light level, which is why colour rendition preferences differ between low and high illuminance.
StandardStrong evidence
Colour appearance of fluorescent materials depends on the ultraviolet content of the light, which the CIE accounts for with a separate ultraviolet metamerism index when grading daylight simulators.
Source: CIE 051.2-1999 A Method for Assessing the Quality of Daylight Simulators for Colorimetry
Colourwise interpretationLimited evidence
For dark pairs, the jump in light level between indoors and outdoors is usually the larger cause of a newly visible mismatch, with spectral change and surface sheen adding to it.
Based on: Reasoned from the roughly hundredfold-plus illuminance difference and the dependence of discrimination on luminance; not measured on specific garments.
Caveat: A strongly metameric pair can mismatch outdoors even at matched light levels.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.