fade_from_range - Gradually fades a part of the palette between two others.
Allegro game programming library.
#include <allegro.h>
void fade_from_range(const PALETTE source, const PALETTE
dest, int speed, int from, int to);
Gradually fades a part of the palette from the source palette to the dest
palette. The speed is from 1 (the slowest) up to 64 (instantaneous). This
routine only affects colors between from and to (inclusive: pass 0 and 255 to
fade the entire palette).
Note that this function will block your game while the fade is in
effect, and it won't work right visually if you are not in an 8 bit color
depth resolution.