Question

why is there a white cloud in the sky ?

Answer 1

While the the cloud whch contains huge amount of water  vapour , reflects most of sunlight as well as absorb it. Thus we get no light through it, so it appears black to us. BlACK cloud is denser so it relatively at lower height. there are clouds in the sky, the light is also scattered by the water droplets inside the cloud. Unlike the gas molecules in the atmosphere, these water droplets are about the same size as the light's wavelength which means that they scatter light in a different way called Mie scattering

Answer 2

when water evaporates and goes in sky it forms whitish structure so clouds are white