Question

why sky is red during sunset?

Answer 1

And as we already know that white light is composed of 7 colours i.e. violet, indigo,blue,green, yellow, orange and red (VIBGYOR).During sunrise and sunset the distance that the light has to travel from the Sun to us is at its greatest. This means most of the light components get scattered so the light that is received by us is mostly of a longer wavelength i.e. Red and therefore appears to be Red. 
That's why  sky is red during sunset.

Answer 2

Within the visible range of light, red light waves are scattered the least by atmospheric gas molecules. So at sunrise and sunset, when the sunlight travels a long path through the atmosphere to reach our eyes, the blue light has been mostly removed, leaving mostlyred and yellow light remaining.