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.