I keep asking myself a question now then why everytime when sunset or sunrise become red while midday is yello?

When you see the sun close to the horizon, it has to travel through much more of the Earth's atmosphere. That extra distance means more and more of it's light is being absorbed - by air particles, pollution, dust, anything else floating around in the air - and the light that you see is more red - the redder, the less energetic the light is.

Overhead, the sun has to go through about 60% *less* atmosphere, and, while it's color can still be tainted by impurities in the air, the distance it travels in the atmosphere is so much less, that we see the light *closer* to it's true color.

Sunset and sunrise are not always red but it is caused by the clouds. Since at sunset and sunrise the sun rays have to travel a further distance, based on the properties of red light(long wavelength) it can travel furhter than yellow light and hence visible as red. At midday, the sun path is relatively short hence a short wavelength(yellow) can pass through.

When the Sun is overhead the light has to travel through only 300 km or so of dusty atmosphere. When the Sun is low on the horizon the light has to travel through 2000 km of dusty atmosphere. The dust scatters shorter wavelength light such as green and yellow leaving the red to illuminate the clouds.

The blue wavelengths of sunlight are scattered by the atmosphere.

The sunlight travels through thicker amounts of atmosphere at sunrise and sunset.

http://www.universetoday.com/77115/why-is-the-suns...

http://www.physicsclassroom.com/class/light/u12l2f...

It's due to the angle that the sunlight is hitting the atmosphere