The particles /water particles present in Clouds scatter light.
Normal clouds having larger water droplets scatter all kinds of light in all directions. This is called Mie Scattering. When we look up at clouds, we see red light, blue light, yellow and orange and green light, all coming at us. We can't see the individual colors, though, because since they're all coming at our eye from the same place. Together, they all combine - Power Ranger style - to make white light. That's what hits our eyes, and makes us perceive clouds as white.