At 12:00 noon, when the sun is at its “highest point” (and I quote that because it is entirely subjective depending on locality), the blue light is scattered by particles in the atmosphere much more efficiently than the red light because blue light has much shorter wavelengths than red light.
At dusk and dawn when the sun is more horizontal to our viewing locations, the sunlight has to travel through much more atmosphere than when it is directly overhead. Instead of coming straight down to us, as it does in a 12:00 vertical position, the light travels horizontally across the atmosphere, which in turn scatters most of the blue light, leaving most of the red light behind.
Obviously, most sunsets aren’t a perfect red hue. When the light passes through the atmosphere, it reflects off of dust molecules, bits of floating ash, pollen, water vapor, and an abundance of other microscopic particles that further scatter and distort the light from its purity. This scattering is through a process called Ray leigh Scattering. It is also the reason why much more colorful sunsets are often seen in more humid locations. The more water vapor that is in the air, the more distortion and manipulation the light will go under before reaching our eyes.
Both the sunrise and sunset are magical phenomena that make our planet Earth unique amongst the other planetary bodies that we know of. It won’t be until we set out and travel the cosmos that we’ll discover other sunsets from other stars and other solar systems. You can think of our sunsets and sunrises as signatures to our life on earth. And you know what they say; no two signatures are exactly the same.