Repainted sky from scratch: sunrise, sunset, greenish tint at noon.

The colours are less artificial and more pleasant.
Also added a GIMP source file with layers for the sky.

Colors based on:
http://facepunch.com/showthread.php?t=1160437&s=422050214f274f7348ed2fea2918b899&p=39591518&viewfull=1#post39591518