You have probably seen a lot of beautiful sunsets. The red sun on the horizon looks great every time. That's why I like photographing sunsets from time to time, even when I already have a lot of pictures of sunsets.

But why does the sun become orange or reddish at the sunset, whereas during the day it is yellowish?

The sun emits the complete spectrum of all colors, which combined is white. That's why the sunlight coming through our windows is white. The emitted light of all colors gets scattered by air particles, which are present even in a clear sky. (According to the laws of nature, blue) gets scattered the most and red the least.

Because of this scattering, the light doesn't follow a direct path from the sun to the camera. Thus, if you photograph the sun during the day, you receive less of the blue light, which is scattered the most. And if you remove one color from a mixture of colors (here the complete spectrum of all colors), you get the complementary color. The complementary color of blue is yellow, so the remaining direct light from the sun appears to be yellow.

During a sunset, the sunlight has to travel a longer way through the atmosphere to us. This light gets scattered more by air particles. Because the red light is scattered the least, this is the color that remains. This is why the sun looks red at sunsets.

A friend of mine told me after his trip to Shanghai that the sun there was red even during the day. This comes from the high air pollution. The light is scattered by dirt particles. Again, only the red light remains because it is scattered the least.

Warning: Be careful, you should never look directly at the sun during the day to check its color. You can observe it only on photos or with special eyewear, as is common during an eclipse. The sun is so strong that it would damage your retina.