用英語的回答：Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air.Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere.Blue is scattered more than other colors because it travels as shorter,smaller waves.This is why we see a blue sky most of the time.
（更深一步的回答：）
sky appears blue because blue color have minimum wavelength due to which it scatters most (see diffraction) as compared to other colors red scatters least thats why it is used in making signals,neon advertisements.though others also contribute to the color of the sky to some extent.
In the evening when sun is at horizon only red and other high wavelength lights manage to reach our eyes,most of the blue scatters in the way and the sky appear reddish.
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