(taken from treebeard)Convert all the peices of a pre existing sky to bmp format
Edit them with paint gimp pixia whatever you use.
Rename them to a new name
convert them to ddt - takes ages lol
Open up SkySunSet or another sky name with a hex editor.
replace the current texture names to your new ones and save it as a new name.
convert skies.xmb and open up the xml with notepad
Put a new tag the with your new sky model name