We've all seen how when we splash water on our clothes, or get caught in the rain, the places where the water hits appear darker than the rest of the cloth. Does the cloth really change color when it gets wet?
No, it doesn't, but something does happen to the white light striking the cloth that makes it seem like there is a color change.
Colors And Cloth
White light, such as sunlight, is made up of a mixture of light of all colors. So when it lands on a pair of blue jeans, for instance, the jeans look blue because when white light strikes the blue threads, mainly blue light is reflected back to our eyes, while most of the other colors are absorbed by the cloth.
When a spot on the jeans gets wet, that area is coated with a film of water, and water fills spaces around the threads. Light striking the wet spot is bent down among the fibers. There it may reflect off the surface of the water, and bounce off the threads several times before it returns to our eyes.
With each bounce more light is absorbed by the cloth, and more non-blue light is absorbed than blue. Eventually some of the light is reflected back to our eyes, and since more of the blue light is reflected by the cloth, what we see looks bluer than the dry material.
Incidentally, the dry cloth looks lighter because there's no water to bend the light down between the fibers where more of it can be absorbed. So, even though there is still more blue light reflected, more of all the colors are reflected to our eyes as well. Since all colors combined make white, it's sort of like mixing white with blue to make a lighter blue.