Why Is the Daytime Sky Blue but Sunset Red?

The daytime sky is blue mainly because sunlight is scattered by molecules in the atmosphere, not because air itself is blue. Among visible wavelengths, shorter blue light undergoes Rayleigh scattering more strongly than longer red light. Some blue light travelling from the Sun is therefore redirected across the sky. When we look away from the Sun, much of the light reaching us is this scattered light, so the sky appears blue. Violet is scattered even more strongly, but sunlight contains less violet and human vision is less sensitive to it.

Near sunset, sunlight travels obliquely through a much longer stretch of atmosphere than it does around midday. Much of the blue light is scattered out of the direct path before the light reaches us, leaving the Sun and the nearby sky looking more yellow, orange or red. The central point is that blue skies and red sunsets are not produced by opposing mechanisms: they are two viewing outcomes of the same scattering process. Dust, droplets and pollution can alter the colour and brightness, which is why sunsets do not all look equally red.

https://spaceplace.nasa.gov/blue-sky/en/