Why do worn jeans turn white first at the spots that rub most?

Answer: Surface indigo rubs off

Surface indigo rubs off ✓ — Right. Indigo dye in denim often sits near the yarn surface, while the yarn core and the weft can remain pale. The spots that rub most lose that outer dye first and reveal the lighter material underneath. The iconic fade is therefore controlled damage, not simply poor dye quality.

Sunlight bleaches sweat — Not quite. Sunlight can fade many dyes, and sweat can affect clothing, but jeans' high-contrast fades usually map to abrasion rather than exposure alone. A rubbed spot can whiten even if it is not the sunniest part of the garment. That pattern fits surface indigo wearing away much better than sunlight selectively bleaching sweaty areas.

Detergent whitens rub spots — Not quite. Detergent can affect laundry brightness, but it does not selectively target the places that rub most during wear. Those spots whiten first because mechanical abrasion removes surface dye. The washer may help carry away loosened dye, but the map of the fade is drawn by rubbing through surface indigo.