Question

why sometimes sky is blue and red

Answer 1

sky is red during early morning and evening because of the fact that , during those time the distance between earth and sun will be more. So only red and its complement colours whose wavelengths are comparatively higher can reach earth travelling long distance.
during day time, the distance is comparatively short. So short wave length light will get scattered more and will be seen as blue. 

Answer 2

Sky is sometimes blue and red because of a phenenon called " Scattering of Lights". Sky appears blue as air molecules scatters blue light more than red at the time of morning. Since blue light has shorter wavelength than red so it scatters in all directions because of tiny molecules. Since blue light was scattered in the morning that's why red light scatters at the time of sunset or evening.