Terragen 2 gives you quite a bit of control over the atmosphere in your scene. In this post, I’ll cover the Main options tab including a video that shows how some of the options on that tab.
In my sample scene (download here), we have set a yellow sun and a smaller red sun several degrees above the horizon, with the camera pointed directly that them. The amount of haze in the environment, the color of that haze, the density of “Bluesky” (non-haze component of the color) and “Bluesky” color (which can be any color… we’ll change it to red) can all be modified. Have a look at what some changes look like:
Notice how the haze settings control how much haze you get and what color it is, while the Bluesky controls manage the color of the sky itself (what’s “behind” the haze, or if you look in a different direction away from light sources) and how intense that color is.
The Bluesky additive and Redsky decay modify the Bluesky density and Bluesky horizon color… they look to be another way of thinking of sky coloration but change the same rendering parameters at the end of the day.
For more information and insight on atmosphere rendering, have a look at the bottom of Page 16 of the User Interface Overview.