Well if you are asking why the sky is blue it is because that is main color of light reflected by the particles of matter in the atmosphere (oxygen, nitrogen, etc.).
Light contains all the colors. Anything is the color it appears because that is the color of light it reflects back. Black means all the colors are being absorbed. White means all the colors are being reflected back. If this is more information than you were looking for than you should think twice before asking a nerdy science teacher a question.