CBSE Class 10 Answered

Clouds are made of tiny droplets of water or ice. When these droplets are of the right size, they scatter all colours of light. Due to these the sky appears blue.

When the clouds are thin, large portion of light gets transmitted and appears white.

But like any objects that transmit light, the thicker they are, the less amount of light transmits through it. So as the thickness of the clouds increases (especially before rain), the bottom of the clouds appears darker but still scatters all colors. We perceive this as grey. Hence rain clouds appear greyish in colour other than white.