Indigo-dyed denim is not like other fabrics. With wear and wash, the deep blue colour slowly turns brighter and the white core of the yarn slowly appears. In other words, it fades.
All denim fades, but it’s much more visible on raw denim jeans compared to pre-washed jeans, which essentially already has been faded.

The reason for indigo’s unique fading property is found in the vat dyeing process.

Indigo starts as a powder. To get it onto the yarn or the fabric, it’s solubilised in water. When the yarn or fabric is pulled out of the dyeing vat, the oxidation process binds the colour to the fibres of the yarn.
The indigo doesn’t reach the core of the yarn, but only binds externally. This gives what’s known as a ‘ring dye’ effect. And that’s what makes jeans fade. As the dye slowly wears and washes off, the undyed core appears.

Even with several dips in industrial indigo dyeing, the indigo pigments stay on the surface of the yarn. As the colour slowly wears and washes off, the undyed core appears. This is true for both natural indigo and synthetic indigo when used in modern-day indigo dyeing.