Why do things look darker when they’re wet? It’s common knowledge that if you spill something on clothing, it will leave a dark patch for everyone to stare at. But maybe the less commonly known fact is that this happens as a result of physics.
More specifically, it is to do with refraction, the way light bends when it hits something. Depending on the material, colour and the molecules an item contains, some lights are refracted, some are reflected and some are absorbed. This is how we perceive the difference in colours between things, different colours absorb light at different wavelengths.
Think about fabric, while it is dry, the amount of light absorbed, reflected and refracted helps us see it for what it is. A spillage of water would pout a barrier between the fabric and light, so light would have to pass through the water first, changing the ratios of light. Less would be reflected as it would hit the water first, and some of what does get through would again be diminished when reflected from the fabric.
The biggest change is when the light reflected from the wet fabric undergoes a process called total internal reflection and bounces back towards the fabric again after hitting the water, so, in total the ratio ends up in favour of absorption, with more overall light being absorbed by the fabric, making it appear a darker shade of whatever colour it originally was. This is why it doesn’t look bad on black, it can’t really absorb any more light.