The sky appears blue due to light refraction off of a number of particles in our atmosphere, blue being of the shortest wavelength and able to scatter further than red, like you see in a sunset. Clouds appear to be white, or shades of gray for the same reason of light scattering due to their high water composition, the water droplets being larger compared to light and acting almost like a reversed prism, the wavelengths all scattering the same. So, in theory, the sky is blue during a nice, clear day, but changes if there is a disruption in its scattering, and clouds change to gray tones with the changes in their water content.