TRY adding the following line
flags = LIGHT;
and see if it makes ANY difference.
(I dont have any skies to test with ATM)
If it does chage the sky's appearance at all,
add these lines to the definition and tweak to suit.
red = 150;
green= 150;
blue = 150;
Always keep the numbers balanced or else you will be adjusting
the images hue. (Unless you want, say, a red evening sky...)
Hope it works, never tried it myself.