Short answer
Because mechanical textile recycling keeps the dye in the fibre, so the colour of the recycled yarn is the average colour of the garments that went in; sorting by shade avoids re-dyeing and gives a predictable result. Only chemical routes that dissolve or break down the fibre can remove the colour, and those are used mainly for polyester and cellulose.
In mechanical recycling, garments and offcuts are shredded and pulled back into fibre, then carded and spun. Nothing in that process removes dye, so the recycled fibre carries its previous colour. The Prato carded-wool industry in Italy, which has recycled wool this way for generations, sorts pre- and post-consumer textiles into colour shades precisely so the resulting yarn does not have to be dyed again; the association describes this as saving water, chemicals and dye. Multicoloured material that cannot be classified is pooled into its own batch, and that is the part that gets dyed — usually to a dark shade that hides the mixture.
When fibres of different colours are blended, the eye does not see a new flat colour; it sees a heather or mélange, with individual fibres still visible at close range. That is a physical mixture rather than a dyed colour, and it behaves like optical mixing: the overall impression averages the fibre colours, but the fabric looks richer and less uniform than a solid dye. Recycled-fibre products often lean into this by choosing heathered greys, oatmeals and mixed dark tones, which tolerate the slight variation between feedstock lots far better than a clean solid pastel would.
Hand sorting by colour is slow and subjective, so automated textile sorters use two sensing channels. Near-infrared spectroscopy identifies fibre composition — wool, cotton, polyester, viscose, acrylic, nylon and their blends — while a separate RGB camera classifies colour and distinguishes single-colour from multicolour items. Fibersort, developed for this purpose, describes exactly that arrangement and frames colour sorting as the step that lets recycled textiles avoid re-dyeing. Colour sorting is only as fine as the classes the operator sets, so a 'navy' bin will still contain a range of navies.
Chemical routes break the fibre down far enough that dyes can be separated. For polyester, depolymerisation to monomers allows colourants to be removed before the polymer is rebuilt: Ioniqa, for example, describes removing residual colour components from its reaction mixture so the recycled PET regains virgin-like clarity, and academic work on glycolysing highly coloured PET confirms that colourants follow the monomer unless an extra purification step isolates it. These routes restore a white or clear starting point, so any colour can be dyed afresh — but they use more energy and chemistry than mechanical recycling, which is why colour-sorting remains the default for wool and cotton.
| Route | Fibres | What happens to dye | Resulting colour control |
|---|---|---|---|
| Mechanical (shred, card, spin) | Wool, cotton, blends | Stays in fibre | Sort by shade; mixed lots dyed dark |
| Chemical depolymerisation | Polyester | Separated from monomer | Near-clear base, re-dye freely |
| Dissolution of cellulose | Cotton, viscose | Must be stripped or bleached | Depends on decolourisation step |
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
In Prato's recycled carded-wool production, textile feedstock is sorted into colour shades so that it does not need to be dyed again; only unclassifiable multicolour material is dyed.
FactStrong evidence
Automated textile sorting can combine NIR spectroscopy for fibre composition with an RGB camera for colour, separating single-colour from multicolour items.
Source: Fibersort technology
FactModerate evidence
In glycolysis of coloured PET waste, colourants discolour the recovered monomer unless an additional extraction or purification step isolates it.
Caveat: Evidence is from PET packaging waste; polyester textiles add finishes and blends.
Source: Viability of Glycolysis for the Chemical Recycling of Highly Coloured and Multi-Layered Actual PET Wastes; Denua technology: PET recycling without compromise
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.