The sky's color comes from sunlight scattering off air molecules, and blue light scatters more than other colors (Rayleigh scattering). At midday, sunlight travels through less atmosphere, so you get a vivid, pure blue. Near sunrise and sunset, light travels through far more atmosphere at a shallow angle, scattering away most of the blue before it reaches you and leaving the warmer oranges and reds that survive the trip — plus more dust and humidity near the horizon scatters light differently too.