I made a little experiment this evening: I have installed FS Skyworld 2004, and tested it briefly with FSX.
FS SkyWorld was a very famous freeware program that let you install a big set of new clouds and sky colors for FS2004. You could choose from more than 10 sets of new cumulus, and even more colors for dawn, day or dusk skies (deep or light blue, purple, red, orange, combination, etc...)
In FSX, it seems that one can install the clouds with no problems at all.
However, it does not work for the sky colors: I get a flashing screen with repetitive design (like when Windows goes bad, and you can see the trails of the window you are dragging, or the trail of your mouse, see what I mean ? ) everywhere the sky should appear. No problems with the ground, bu as soon as you look up, the graphics go crazy

However, the error was repaired by re-pasting all the original sky*.* textures in the Texture folder of FSX.
So, remember: make a BACKUP OF YPOUR ORIGINAL FSX "TEXTURE" FOLDER FIRST !!!
Here is a little screenie of my experiments. Could anybody confirm this is working ?
and from another angle:
BTW, this is Alpnach Mill at dusk, beautifull area in FSX
