fade_in_range - Gradually fades a part of the palette from black. Allegro game
programming library.
#include <allegro.h>
void fade_in_range(const PALETTE p, int speed, int from, int
to);
Gradually fades a part of the palette from a black screen to the specified
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.