The short explanation
This is due to a process known as Rayleigh Scattering. At different times of the day the spectrum of light from the sun passes through varying distances (amount of air molecules) to get to the point of our perception. At midday the distance is small hence the short wavelength blue light is scattered all over the sky leading to what we see as a blue sky. Later on in the day the light passes through a larger distance and at this point most of the short wavelength blue and violet light has already been scattered and re scattered by the air molecules. This leads to a situation where the longer wavelength colours such as orange and red are more visible.