Freshly indigo-dyed jeans yarn can look yellow before blue. What flips it?

Answer: Air restores blue indigo

Air restores blue indigo ✓ — Right. Indigo enters the dye bath in a reduced, water-soluble leuco form that is yellowish rather than blue. Once the yarn comes back into air, oxygen oxidizes it back into insoluble blue indigo. The strange part is that jeans are not simply dipped into blue paint; the famous color is partly made during the drying gap outside the vat.

Steam fixes yellow dye — Not quite. Heat and steaming can matter in textile finishing, but the yellow-to-blue moment in indigo dyeing is an oxidation reaction, not a steam-setting trick. ChemMatters describes leuco indigo becoming blue again when exposed to oxygen. Steam would not explain why the yarn changes color just by meeting air between dye baths.

Cotton shrinking darkens — Almost, but cotton shrinkage is a fit and fabric-structure issue, not the color switch. A yarn can change shade before any garment washing or shrink-to-fit ritual has happened. The key visual change is chemical: leuco indigo oxidizes back to blue indigo when it meets air.