fade_in_range - Man Page
Gradually fades a part of the palette from black. Allegro game programming library.
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.
version 4.4.3 Allegro manual