What color is the sky really?
a. blue
b. green
c. red
d. oarnge
e. all of the above

I suppose it comes to the time of day, but technically I would suppose (E: All of the Above)

Explanation:

The color of the sky is due to a phenomena called Rayleigh scattering.  In Rayleigh Scattering, light waves travel through a medium (in this case, the atmosphere) and strike the individual particles which bend the light.  The light waves from the sun are smaller than the particles in the atmosphere (the molecules and the atoms in our atmosphere) and are bent when they strike these particles, resulting in the color of the sky you see at whatever time.  I'm no physics major, but this is one of the major reasons we see the sky in the way we do.  If you wanted to go further, you could even go and say that the Martian atmosphere is red because of the exact same phenomena.

Did you Know?  A Martian sunset is blue, check it out: https://www.bing.com/images/search?q=martian+sunset&FORM=HDRSC2