The color of a cloud depends primarily upon the color of the light it receives. The Earth's natural source of light is the sun which provides 'white' light. White light combines all of the colors in the 'visible spectrum', which is the range of colors we can see.

Haze and dust in the atmosphere can cause clouds to appear yellow, orange, pink or red. And as clouds thicken, sunlight passing through the cloud will diminish or be blocked, giving clouds a grey color. If there is no direct sunlight striking the cloud, it may reflect the color of the sky and appear bluish.