oh!! so i am definitely not an expert but from my observations ( mainly looking at references ) stuff in the background are usually more blue and faded :O

here's an explanation from google: 

Air is not entirely transparent. A small portion of the light gets scattered away from the straight path it would take in a vacuum and thus does not arrive at a viewers eye. The longer the distance the more light its affected. In most areas you also have more our less other stuff in the atmosphere. ( wow thanks google that definitely helps a lot )

Sky light usually contains more light of short wavelength than other wavelengths (this is why the sky usually appears blue), which is why distant objects appear bluish.

you can see that in these pictures:

the mountains in the back are more blue and a lighter color! the father they are, the more blue and faded they become :0

you can see that in this picture too :o

things farther away will also have less detail!

i think the best way to make it look real is to just use references :00 its makes it much easier when you can pick the colors from a image!

hope this helps!! ;w; again i'm not an expert so my information may not be correct :'DD if you want to know more you can ask someone else or google it!