The light coming from the sun is made of many colors; light travels as a wave, and each color has a unique wavelength. Violet and blue light has shorter wavelengths, while red light has a longer wavelength, and the other colors have wavelengths in between.
When the different colors of light pass through the atmosphere, they run into molecules, water droplets and bits of dust. Because all these particles are closer in size to shorter wavelengths of light, they tend to scatter violet and blue light much more than red, and so they send rays of violet and blue ricocheting toward the ground and your eyes. More violet light actually gets scattered by atmospheric particles than blue light, but your eyes are more sensitive to blue, so the sky appears blue.
Sunsets are orange-red because in the evening, with the sun low on the horizon, sunlight must pass through more atmosphere to get to your eyes, and only the red light can make it all the way through. The shorter wavelengths have all been scattered toward the ground in the part of Earth where it is still daytime.
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