Refractive Index

Light travels at different speeds through different materials, and the refractive index is that comparison as a figure. Air is about 1, water about 1.33. Most cosmetic oils sit near 1.5, and so does the keratin of skin's outermost layer. On its own each number is inert; what matters is the gap between two of them where they meet.

At a boundary with a large gap, some light bounces instead of going through. Many boundaries mean many bounces, and light that keeps bouncing near the surface never reaches the pigment underneath, so it comes back carrying none of the skin's own colour. That is what dull, grey and washed-out describe optically. Not less colour — less of it getting out.

Dry skin is full of such boundaries. When surface cells lift instead of shedding cleanly there is air between them, and air against keratin is close to the largest mismatch skin can offer. Wet the surface with something of a similar index and the gaps fill, the mismatch shrinks, and the colour returns. Same reason a wet pebble looks richer than a dry one.

Formulators pull on this deliberately. Matching a particle's index to the medium around it makes a dispersion look clear; mismatching it makes a white. Mineral filters are high-index particles, and their visible cast and their UV attenuation come from the same particles — which is why the answer has been to make them smaller, not to make them vanish.