fade_from (3) - Linux Man Pages
fade_from: Gradually fades the palette between two others. Allegro game programming library.
NAMEfade_from - Gradually fades the palette between two others. Allegro game programming library.
DESCRIPTIONFades gradually from the source palette to the dest palette. The speed is from 1 (the slowest) up to 64 (instantaneous).
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.