Raleigh scattering is when molecules deflect electromagnetic radiation which has a larger wavelength than the size of those molecules. When white light containing all wavelengths (figure 2) comes in contact with air molecules in the Earth's atmosphere, shorter wavelengths are scattered. All other wavelengths of light are not scattered therefore pass through the atmosphere still mixed together so are percieved as white light. Those shorter wavelengths which are separated in the atmosphere give the sky a blue colour (figure 1).

I'm pretty sure that during sunset/ sunrise, the Earth is at a steeper angle to the sun compared to during the day (figure 4) meaning sunlight passes through more of the atmosphere causing an increased scattering of shorter wavelengths. This results in all visible wavelengths getting scattered apart from the oranges and reds (look at figure 2 again) causing that sky in sunrises. Same kinda thing happens at the horizon where blue gets scattered more but since sun is still up more white light makes it through causing horizons appear a lighter blue than the sky (figure 5).