Sunlight entering our atmosphere is scattered by Oxygen and Nitrogen molecules. As these molecules are much
smaller than the wavelengths of visible light, they scatter smaller wavelengths more efficiently than longer ones (this is called
"Rayleigh scattering"). Violet and blue have the shortest visible wavelengths so those colors will dominate the scattered light. As our
eyes are more sensitive to blue than to violet, we perceive the sky as "blue".
Clouds appear "white" because their water droplets are much larger than visible wavelengths.
When the reflectors are larger than the wavelengths being reflected the efficiency is nearly independent of wavelength (this is
called "Mie scattering"). Since all colors are uniformly reflected, clouds appear "white".
Spectrum of Blue Sky