The blue color is caused by some of the blue light from the spectrum of sunlight being scattered by two processes, Rayleigh scattering and Mie scattering. Both processes involve atoms, molecules, and particles that are smaller than the wavelength of blue light. If our atmosphere was made up of larger molecules, the sky would be a different color. The intensity of the scattering of blue light does not really change much from day to day, but it can be hidden by larger particles, such as water droplets, which scatter all the colors of light. That is why clouds are white. If the atmosphere has lots of particles that are scattering all of the light, that white light lightens the intensity of the blue. Clean, dry air often gives you bright blue skies, while dusty, humid air often produces skies that that seem to have less blue. Actually, they don't have less blue, just more of all the other colors.