Short answer
Dyed textiles are usually coloured with mixtures of several dyes, which gives them bumpy, irregular reflectance spectra. A lamp with peaks and gaps in its own spectrum samples those bumps unevenly, so the balance of the colour shifts: a taupe or olive loses the red that kept it warm and turns greenish, or a navy turns purple. Optical brighteners add a second effect: they need violet or ultraviolet light to glow.
A paint can often reach a colour with a few broad pigments. Textile dyers typically build shades from mixtures of dyes — often three or more — chosen for cost, fastness and fibre compatibility, and each dye contributes its own absorption band. The resulting reflectance curve can have a secondary rise in the red, a dip in the yellow-green and so on, even though the fabric looks like a single calm colour in daylight. Muted and neutral shades — taupe, khaki, olive, grey, stone, brown, navy — are built from several dyes that nearly cancel each other, which leaves them especially exposed to a change of light: small shifts in the balance show as a hue change on an otherwise quiet colour.
Many lamps put strong power in the green-yellow, where the eye is most sensitive, and comparatively little in the deep red. Fluorescent tubes add a mercury line at 546 nm; basic LEDs have a phosphor hump centred in the yellow-green and a weak red tail. A warm neutral fabric that owes its warmth to a secondary reflectance rise above about 620 nm loses that contribution under such a lamp, while its green-reflecting component is fully lit. Once your eyes adapt to the lamp's white, the fabric's balance has moved towards green relative to everything else. Swap to a lamp with a strong deep red and the same fabric can swing the other way, towards pink or brown.
The table on the R9 page shows the same mechanism with a saturated red: under the halophosphate fluorescent it loses some chroma and swings roughly 16° in hue towards orange relative to daylight.
White and pastel textiles, and many detergents, contain fluorescent whitening agents that absorb ultraviolet and violet and re-emit blue, cancelling the natural yellowness of fibre. Outdoors, daylight supplies plenty of ultraviolet and whites look brilliant; behind window glass or under a blue-pumped LED, which emits almost nothing below about 420 nm, the effect largely switches off and the same shirt looks creamier. The CIE recognises this in its method for grading daylight simulators, which has a separate ultraviolet metamerism index for fluorescent samples. Under a violet-pumped LED or a black light the effect returns, sometimes to an unnatural blue glow.
A suit jacket and trousers bought separately, or a blouse with a contrasting trim in 'the same' colour, are often dyed in different lots or on different fibres — wool and polyester take different dye classes entirely. They can match perfectly in the shop and part company under a restaurant's warm LED or an office's fluorescent tubes. The only reliable check is to look at both pieces together under at least two very different lights before committing, as the site's fabric guides recommend for pairing.
Why: Warm LED or fluorescent light lacking deep red removes the red component that made the taupe warm.
Fix: Check swatches under your own evening lamps before ordering; a lamp with higher R9 reduces the shift.
Why: Different dye lots or fibres, metameric under the office's fluorescent or LED light.
Fix: Buy suit pieces from the same lot, or check both under office-type lighting before buying.
Why: Optical brighteners fluoresce only when violet or ultraviolet light is present.
Fix: Nothing is wrong with the shirt; indoor LEDs simply do not excite the whitener.
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.
FactStrong evidence
Fluorescent lamps emit a strong visible mercury line at 546.1 nm in the green, superimposed on their phosphor emission.
Source: NIST Handbook of Basic Atomic Spectroscopic Data: strong lines of neutral mercury
StandardStrong evidence
The CIE's daylight-simulator assessment includes an ultraviolet-range metamerism index specifically for fluorescent samples, because their appearance depends on ultraviolet content.
Source: CIE 051.2-1999 A Method for Assessing the Quality of Daylight Simulators for Colorimetry
Colourwise analysisModerate evidence
The CIE's halophosphate fluorescent FL2 and phosphor LED LED-B3 illuminants both carry a smaller share of deep-red (640–730 nm) power than daylight D65.
Based on: Calculated by Colourwise from the CIE tables sampled at 10 nm.
Caveat: Power share, not visual effect; real lamps vary.
Source: CIE datasets (colour-matching functions, illuminants)
Colourwise interpretationLimited evidence
Muted, near-neutral textile shades are the most prone to visible hue shifts between lights because their colour is the small residue of several dyes that nearly cancel.
Based on: Follows from the mechanism of illuminant metamerism applied to multi-dye recipes, and from the site's observation that near-neutrals shift most in its room lighting guides; not a measured survey of fabrics.
Caveat: Individual recipes vary; some saturated shades are also highly metameric.
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.