From OHRRPGCE-Wiki

I believe Final Fantasy 4 simply drastically reduced all red and green values (set them to 1/8th of what they were or something like that) so that primarily the blue values of each color remained, and that produced a pretty good nighttime effect. You could do this by:

variable (i)
for (i, 0, 255) do, begin
   write color (i, color:red,   read color(i,color:red)/8)
   write color (i, color:green, read color(i,color:green)/8)
end

Tweak that (by changing the 8's to other values) until you get the effect you want.

This would work better than a simple "tweak palette (-48, -48 , 0)" because if a color has a red or green element less than 48, it'd go to 0, which would lose much detail.

[edit] See Also

• How do I make gradual fade in and fade out?