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Why a Black Hoodie Looks Different in Every Light

Two blacks that match indoors can split apart in October daylight. What metamerism is, why dye and lamps cause it, and how to check before you buy.

Two black hoodies on the same hook. Under the hall light they are the same black. Take them both outside at eight in the morning in late September and one has gone slightly brown, the other has a cold blue cast. Nothing faded. Neither was washed. The light changed, and the two cloths disagreed about it.

Autumn is when people notice this, because autumn is when the light moves fastest. The sun sits lower, the days are shorter, and more waking hours happen under lamps. A black bought in July under shop lighting gets its first proper look at daylight in October. Here is what is going on, in the vocabulary the colour trade uses.

The word for it is metamerism

The International Commission on Illumination, the CIE, maintains the standard vocabulary for light and colour. Its International Lighting Vocabulary defines metamerism as the "property of spectrally different colour stimuli that have the same tristimulus values in a specified colorimetric system". The companion entry for metameric colour stimuli, published in December 2020, puts it from the other side: spectrally different stimuli that land on the same tristimulus values, also called metamers.

Plain version. Two cloths can reflect light in different patterns across the spectrum and still add up to the same colour in your eye, so long as you look under one particular light. Change the light and the sum changes, because the two were never reflecting the same thing to begin with. They agreed by arithmetic, not by chemistry.

Worth being precise. This is a relationship between two objects, not a fault in one. A single black hoodie cannot be metameric on its own. It becomes a problem next to a second black that reached the same answer by a different route.

Two lights, two answers

Different sources put out different amounts of energy at different wavelengths, which is why the lamp can change the verdict. The CIE fixes reference points so results can be compared. Its entry for CIE standard illuminant gives illuminant A as Planckian radiation at roughly 2 856 K, standing in for tungsten filament lighting, and illuminant D65 as daylight at a correlated colour temperature of about 6 500 K.

Both are defined in ISO/CIE 11664-2:2022, Colorimetry, Part 2: CIE Standard Illuminants, which sets out relative spectral power distributions for A, D65 and D50 at one nanometre intervals from 300 nm to 830 nm. D50 was added in the 2022 revision for graphic arts and photography.

D65 is a defined average, not a description of the sky over Winnipeg on a given Tuesday. Real daylight moves around it. The standards exist so two labs comparing a cloth are at least arguing under the same light, which is more than can be said for your porch and the changing room.

Shop lighting is the awkward part

The US National Institute of Standards and Technology, in its material on colour rendering of light sources, says the colour appearance of objects under artificial lighting is an important characteristic of a light source, then sets out where the usual metric falls short. NIST notes that the CIE Colour Rendering Index uses too few reflective samples, that those samples have too low chromatic saturation, that the colour space behind it is outdated, and that averaging the differences across all samples lets a lamp score well while rendering one or two colours badly.

Carry that last point into a shop. A ceiling of lamps with a respectable headline rating can still be poor at exactly the part of the spectrum that separates two blacks. The rating is an average. Your problem is not average.

Dye is where it starts

Two blacks split because they were made black by different means. Cotton Incorporated's technical bulletin ISP 1015, Chemistry of Fiber Reactive and Reactive/Disperse Dyes in the Printing Process, from 2006, describes the two colorant systems used on cotton and on cotton-polyester. Fibre reactive dyes react with the cotton fibre through covalent bonds under set conditions of pH, temperature and time. Disperse dyes are not water soluble. They are dispersed into a synthetic fibre by heat, and the bulletin classes them as low, medium or high energy according to the sublimation temperature needed to fix them.

So a black in a cotton-poly hoodie is generally two colorants doing two jobs on two fibres in one cloth. A black in an all-cotton hoodie is one system. Different spectral signatures, same word on the label, which is a large part of why a blend and a straight cotton piece so often refuse to match away from the light they were bought under. The fibre side of that argument is in cotton or cotton-poly for a heavy hoodie.

Black shows this worst, and the reason is arithmetic again. Black cloth returns very little light, so a small absolute difference in what it returns is a large share of the total. That reasoning is ours rather than a finding from the sources above, but it matches the experience of anyone who has fought to match two blacks and never had the same trouble with two greys.

How the trade settles it

Formal procedures exist because judging colour in the wrong conditions is unreliable. ASTM International publishes ASTM D1729, Standard Practice for Visual Appraisal of Colors and Color Differences of Diffusely-Illuminated Opaque Materials, which specifies equipment and procedure for appraising colour under diffuse illumination and requires observers with at least normal colour vision. Its scope excludes metallic and pearlescent finishes, which need light closer to a geometric simulation of sunlight. AATCC publishes Evaluation Procedure 9, Visual Assessment of Color Difference of Textiles, now in its 2025 edition, standardising how a specimen is compared by eye against a standard.

Describing those is not claiming them. Nothing here says any garment has been through either one. The point is narrower. When colour has to be judged rather than guessed, the light, the backing and the observer get specified first, because all three change the answer.

What actually helps when you buy

  • Judge a black against what it has to live with. A hoodie on its own tells you nothing. Hold it against the jacket or the trousers it will be worn under.
  • Look at the pair in daylight before the tags come off. A window beats nothing, though tinted or coated glazing filters the light, so it is not the same as stepping outside.
  • Then check under the light you live in. Kitchen LEDs, the car at night, an office ceiling. Hold across two or three of those and it will hold.
  • Do not settle it with a phone photo. Automatic white balance rewrites the frame, and two blacks in one shot can be pushed closer or further apart than they are.
  • Buying a second piece in the same colour months later is a different gamble from buying two at once. Different lot, possibly different cloth.
  • If a piece only ever has to work indoors, none of this matters. Layering is where it bites, which is the case covered in how to layer a heavy hoodie for a Canadian autumn.

What none of this settles

Metamerism is not fading. Fading is dye loss, it runs one way, and it does not reverse when you walk back indoors. A metameric mismatch is there from day one and comes and goes with the light. A black that has gone rusty in daylight and stays rusty under every other lamp is a different problem with a different cause.

There is also no reading this off a label. Fibre content hints at which dye systems were involved. It is not an answer. Knowing whether two specific pieces will part company under a specific lamp takes spectral measurement of both, and nobody is doing that in a changing room.

Some common advice is untested rather than wrong. Washing a new black once so it "settles" is the usual suggestion. A first wash does remove loose surface dye and mill finish, and it changes how a cloth reads. Whether it resolves a mismatch between two garments is not something we found tested anywhere, and we will not claim it does. Assume the mismatch you see on day one is the mismatch you keep.

Black is not one thing. It is a destination several chemistries reach by different roads, and October daylight is good at showing which road a cloth took. Our colours are on the shop page. The only real test is the one you do at a window.

Sources

PreviouslyShould You Wash a Hoodie Inside Out?
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