why the sun color become yalow at sunrise and sunset only?

Answer 1

The sun is red/yellow at sunrise and sunset for the same reason that the sky is blue. Tyndall scattering is proportional to the fourth power of the frequency, so the shorter wavelengths (blue, green) are scattered more than the longer yellow, red wavelengths. At sunrise and sunset the path length through the atmosphere for direct light from the sun is at a maximum, and so is the Tyndall scattering. Proportionately more of the solar radiance at blue/green wavelengths is scattered from the direct path so the sun appears yellower/redder.