Clouds are white because the size of their ice crystals or water droplets is big enough to scatter the seven wavelengths of sunlight that produces white light. With all of the colors dispersed, the human eye perceives the complete color spectrum, which combines to make the color of clouds white.
Clouds consist of water vapor formed into droplets that are bigger than air molecules and large enough to be visible. They form as water vapor rises in the sky and the cooler air temperature causes ice crystals or water droplets to form and bond with particles in the air, such as dust or salt, according to NASA.
The water droplets in clouds are large enough that they are able to scatter through all the wavelengths of the colors of the rainbow: red, orange, yellow, green, blue, indigo and violet. Sunlight scatters more than once when it hits each droplet of the many in each cloud, so by the time the light reaches the human eye, there is no dominance of a specific color, as described by Science Niblets. The combination of the seven wavelengths that form the complete color spectrum produces white light, which gives clouds their milky color.