I thought Irish answered your sky question, more or less.
In a sunset i did recently, it was a matter of hand adjusting the "zenith-to-horizon" color blend in the "Sky" tab color blend map called "Sky Color Map" in the Atmosphere editor.

This was an atmosphere where i had set the lighting to Global Illumination.

Right-click on the blend map. Double-click on the little "Keycolor" circle controls to change colors, drag them to blend, click inside colors to make a new Keycolor.

The 2 "Color map position on sky dome" sliders need to be set to minimums for the colors to begin at the horizon and change rapidly. Hope that's what you're looking for.

_ jc...'Art Head Start' e-book
.......'Art Head Start.com site Digital Art skills. Free lighting chapter, tutes, Vue models, tex pix.