The sky is blue because blue light gets scattered (spread) around much more than all the other colors from the sun, causing the sky to appear blue.
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Clouds are white because water and ice in clouds scatter all color wavelengths by the same amount. All colors are represented in this scattered light and clouds therefore appears white. (White light is made up of all colors, red, blue, violet, etc.). All wavelengths of light get scattered equally by drops of water and small clumps of ice.
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