Clouds consist of millions of tiny water droplets.
Light waves of all the colors bounce and reflect off and refract through the water droplets evenly, sending all of the various colors off in (more or less) equal measure.
As you may remember from school, the “color” white is technically all of the colors mixed together.
Because all of the colors are being evenly dispersed by the water droplets, clouds appear white. We think this is kinda cool, because if clouds weren’t white, the sky would be blue all day and everyday, forever. And that would be pretty boring.