However, when the sun's rays hit these particles, they appear white because of diffuse reflections. This is similar to the way that snow looks white when viewed through a pair of magnifying glasses.
By the way, when it rains, the clouds are right above us. We are looking at the bottom of the cloud. In that cloud, there are many raindrops that are about to fall. Moreover, these rain clouds are so thick that the sun's rays cannot penetrate from the top to the bottom of the cloud. This is why the rain clouds we see when we look up at them appear black.