Why the sun seems to be red or yellow at different parts of the day?

The true light that leaves the sun is white.  That is, it contains
ALL colors of light.

Whenever you see the sun ... OR see anything that's lit up by
the sun ... the light you see has passed through the atmosphere. 
The light passes through the LEAST  amount of atmosphere at
Noon, and the MOST of it at sunrise and sunset.

The amount of atmosphere the light must pass through determines
which wavelengths are absorbed (soaked up, subtracted), and how
much.  When the light finally reaches where we are, we see what's
left of it.

Answerwavelength;