Why are clouds normally white or darker in a rainy day? Why the sky looks blue? Here's the story:

Clouds are made up of millions of tiny water droplets & ice crystals. These particles are incredibly small, about 10 to 20 micrometers in diameter, which is roughly the size of the wavelengths of visible light. When this sunlight hits the tiny water droplets in the cloud, it gets scattered in all directions. This scattering is called Mie scattering, it treats all the colors of sunlight equally. So, instead of scattering just the blue light (like what happens with the sky), it scatters all the colors together making them appear white.

On a rainy day, clouds look gray or dark because when clouds accumulate more water droplets they become more efficient at absorbing & blocking sunlight. Instead of the sunlight passing through and scattering evenly, as it does in thinner clouds on sunny days, the water droplets in rainy clouds absorb more light. This absorption makes the clouds look denser & darker from the ground.