Have you ever noticed that when a shirt gets wet, it becomes more transparent? This phenomenon is caused by the way light interacts with the fabric. When water or other liquid soaks into the fabric, the spaces between the fibers are filled with it. This results in a smaller change in the angle of the light incident on the material, leading to greater internal scattering. As a result, less of that light returns to the eyes.
Let's take a closer look at why we can't see through people's clothes. There are two main reasons for this. Firstly, we put dyes on them, which absorb certain colors of light and allow others to reflect. However, this is not true for white clothes such as cotton.
The reason why we can't always see through people's clothes to their underwear is because clothes are made of fibers that scatter light. This is similar to how milk looks white due to tiny particles called casein, which are about the same size as the wavelength of light. Light scatters strongly against these particles in all directions, making it impossible to see through milk. Cotton is made of many fibers around the same size as the wavelength of light, and thus we can't see through it either.
When cotton gets wet, there is water around all those fibers and the light no longer scatters very strongly. This makes the material more transparent and thus more visible. Wetting a stain on your shirt doesn't make that stain permanently darker or inherently different. Water only makes the fabric appear darker by making it more transparent, allowing you to see darker objects behind the fabric.
It really has to do with how transparent the shirt is, but even then, bleached cotton tends to be more translucent than dyed cotton. T-shirts also tend to be more transparent than other shirts unless they are made from very transparent cotton ring spinning. If you hold a white shirt with a damp stain right in front of a bright light, the wet spot will appear brighter than the rest of the shirt because it is more transparent and lets in more light. As a result, most of the white light that falls on an undyed shirt eventually reflects away from the shirt randomly. Because the fabric is fine, it looks totally clean and textureless from a distance, while the twill gives it that hint of shine that gives a white shirt a noticeable shine.
This means that a dry blue shirt is actually whitish blue (which we call lighter blue), but when wet it appears simply blue (which we call darker blue). A white t-shirt, for example, is composed of fibers that are mostly transparent but in such large quantities and concentrations generate a vibrant white color.
