Why does the sun appear red at the time of sunrise and sunset?

The Sun doesn't really change color, it just appears that way. When the Sun is lower, closer to the horizon, the white light has to travel through more of the Earth's atmosphere. Refraction scatters the blue end of the spectrum more (which is why the sky is blue). The red color of light is scattered the least, and that is what you see on the horizon and in clouds.

Smoke, as from forest fires, makes the Sun look red for the same reason. The ash clouds from the Mount St. Helens volcanic eruption provided some of the most beautiful sunsets for a long time afterward.