This mod performs one basic function: to change the sky and fog color of the overworld or nether. For the overworld, it also removes the sun, moon, stars, and clouds. The command to do so is:
/skycolor [dimensionID] [r] [g] [b] <delay>
This color is saved to the loaded world, and transmitted to all connected players.
The dimension ID of the nether is -1, the overworld is 0. The R, G, B values can accept floating-point values, so anywhere between 0.0 and 1.0 is probably what you'll want to use, though you can go higher for a more fog-encroaching effect. The delay is optional, and is the number of milliseconds for the transition to complete. The default is 5000 (5 seconds).
To disable for a dimension, set any of the R, G, B values to be negative.
This mod was designed for use in the Infernal Skies modpack but you're welcome to use it wherever else you see fit.