Why Does Wet Fabric Look Darker?

A pale blue fabric often develops a dark blue patch when it is wet, but the water has not made the dye darker. We see an object because some incident light is returned to our eyes. Dry fabric contains air between its fibres. Because the refractive indices of air and fibre differ substantially, light repeatedly changes direction at many tiny interfaces, and a relatively large share is scattered back out, making the surface look brighter.

When water fills those spaces, its refractive index is closer to that of the fibres than air is. The interfaces scatter less light back, so more light travels into the material instead of returning immediately to the observer. The longer path also gives the fibres and dye more opportunity to absorb the light. With less diffuse reflection reaching the eye, the colour looks darker and more saturated. A surface film can also produce a bright, directional reflection, so a wet material is not necessarily darker from every viewing angle.

Comparing the wet and dry regions of the same cloth under the same lighting isolates this change in optical state. The colourant has not changed; water has altered how light travels, scatters and is absorbed among the fibres. As the cloth dries and air returns to the gaps, its usual appearance generally returns as well.

https://opg.optica.org/ao/abstract.cfm?uri=ao-27-7-1278
https://ece.ncsu.edu/2024/why-do-things-look-darker-when-theyre-wet/