Why Are Some Clouds Darker Than Others?

Some clouds are thin and others are thick. Because sunlight easily passes through thin clouds, they appear bright or white. Light can't pass through thick clouds so easily. The parts where light passes through will appear white, but the thick parts where the light doesn't pass through will look grey or black. Clouds have many colors depending on their thickness and the angle at which light strikes them. Rain clouds appear dark because they are so thick that red light does not pass through them. In good weather the red light from the sun's spectrum reaches our eyes, making the sunset appear red.