These three facts are closely connected. The sky is blue, the Sun often looks yellow, and the sunrise or sunset appear orange or red. It is tempting to think these are separate phenomena, but they all come from the same cause: sunlight passing through Earth’s atmosphere.
Sunlight is what we usually call white light. This does not mean it has no color. It means that it contains a broad mixture of visible wavelengths. When that light travels through empty space, all those wavelengths move together without being sorted. But Earth is not surrounded by empty space. Before sunlight reaches our eyes, it must pass through the atmosphere.
The atmosphere is made of molecules, tiny particles, and occasional droplets. Light interacts with them. The important point is that not all visible wavelengths are scattered equally strongly. Shorter wavelengths, toward the blue end of the spectrum, are scattered much more efficiently than longer wavelengths, toward the red end.
This is the key to the whole story.
When we look away from the Sun and toward the sky, most of the light reaching our eyes has not come directly from the Sun. It has been scattered into our line of sight by the atmosphere. Since blue light is scattered much more strongly than red light, the sky appears blue.
The Sun itself is a different matter. When we look directly at it during the day, we are seeing sunlight that has come to us more directly. But even then, the atmosphere has already removed some of the blue light from the beam by scattering it away in other directions. What remains is therefore slightly depleted in blue, which shifts the Sun’s apparent color toward yellow.
Outside Earth’s atmosphere, the Sun would not look yellow in the same way. It would appear much closer to white, because the full mixture of wavelengths would reach the eye more directly. This naturally raises the question of why so many stars appear as white dots in the night sky. The answer is that stars do not all begin with the same color. Their intrinsic light depends on their surface temperature, with hotter stars being bluer and cooler stars redder. Earth’s atmosphere then modifies that light by scattering blue wavelengths more strongly than red ones, and for some stars this can shift the visible balance closer to white. But just as important is the fact that starlight is faint. At such low intensities, our eyes do not register color nearly as well as they do in daylight. Cooler redder stars are also often less luminous, which makes their color harder to resolve at large distances. As a result, many stars that are physically bluish, yellowish, or reddish are perceived simply as white points of light.
The red color of sunrise and sunset comes from the same effect, but much more strongly. When the Sun is low on the horizon, its light must travel through a much greater thickness of atmosphere before reaching us. Along the way, a much larger fraction of the blue and green light is scattered out of the beam. The light that finally survives the journey is therefore richer in the longer wavelengths, which is why the Sun and the surrounding sky can appear orange or red.
This also explains why sunsets are not always the same. Dust, water vapor, pollution, and clouds can modify the scattering and absorption, sometimes making the colors deeper and more dramatic, sometimes washing them out. But the underlying mechanism remains the same: as sunlight passes through more atmosphere, the shorter wavelengths are removed more effectively than the longer ones.
In that sense, the blue sky, the yellowish Sun, and the red sunset are not three different mysteries. They are three different views of the same process. The atmosphere is continuously redistributing sunlight, and what we see depends on whether we are looking at light scattered into the sky, light coming more directly from the Sun, or light that has crossed a long atmospheric path near the horizon.
See also: Why do things have color?