The sun does not really change colour as it moves through the sky during the day. The change in the colour that you observe is caused by what happens to the sunlight as it moves through the atmosphere to the earth.

Light from the sun has to move through the air before it reaches your eyes. The atmosphere is full of gases, dust, smoke and other pollutants.

Sunlight consists of the seven colours of the rainbow. When light enters the atmosphere, some of it reaches the ground without bumping into any of the gas molecules or other impurities. This light will reach the ground as white light.

Some of the light however, will bump into the molecules. This light is then scattered: this mostly affects the light with the lower wavelengths. That means that violet and blue light are scattered most throughout the sky. The sunlight that reaches your eyes has the colours with the longer wavelengths, namely, red, orange and yellow.

When the sun is near the horizon, the sunlight must pass through the thick air layer that is just above the ground, parallel to the ground towards the horizon.

This means that the light rays from the sun come into contact with many more gas molecules, dust and pollutants than when it is higher up in the sky. This causes even more of the shorter wavelength blue and violet light to be scattered and only the unhindered light with the longer wavelength, red, orange and yellow will reach your eyes. So at sunset and sunrise you will see the sun as a red ball.

This also causes the skyline around the sun to turn red in colour during sunrise and sunset.

Another interesting fact is that in space, the colour of the sun is white and not yellow as we see it on earth. It is because space is a vacuum, with no molecules to scatter the sunlight. So you only see the pure white light as given off by the sun.