makeacol32 man page
makeacol32 — Converts an RGBA color into a 32-bit display pixel format. Allegro game programming library.
int makeacol32(int r, int g, int b, int a);
Converts an RGBA color into a 32-bit display pixel format, which includes an alpha (transparency) value. There are no versions of this routine for other color depths, because only the 32-bit format has enough room to store a proper alpha channel. You should only use RGBA format colors as the input to draw_trans_sprite() or draw_trans_rle_sprite() after calling set_alpha_blender(), rather than drawing them directly to the screen.
makeacol(3), set_alpha_blender(3), set_write_alpha_blender(3)