this Is'nt as hard as it sounds all you do is open the skys tga files in wally, now click on the lighten/darken tool, and color the skies, to make new ones-but darker ones, now save these.
Now with the new skies created just make a loading point in your map to load another map, this time it will load the other map with the new sky
or if you'd like---The Same Sky, Only darker.
If you don't now how to use the newly created skys, just put them
in the "valve/gfx/env" file, or in "yourmods/gfx/env" file
If you put the skys in your mods folder:
You will create a map with Hammer but when compiled it will not go in, it will just crash.. But don't worry this is because the sky is not in the valves folder--just your mods.
Instead try compiling your map With the "Dont Run Map"Box Checked.
Now load up half-life with this at the end of the target field
" -game yourmodfolder +map yourmapname"
In my case it would be
"-game BJ +map map1"
Now your sky will load fine because It is looking in your mods directory
(-game yourmodfolder) and there you have it.