osx man page for set_color_depth

set_color_depth(3alleg4)					  Allegro manual					  set_color_depth(3alleg4)

NAME
       set_color_depth - Sets the global pixel color depth. Allegro game programming library.

SYNOPSIS
       #include <allegro.h>

       void set_color_depth(int depth);

DESCRIPTION
       Sets  the  pixel format to be used by subsequent calls to set_gfx_mode() and create_bitmap(). Valid depths are 8 (the default), 15, 16, 24,
       and 32 bits. Example:

	  set_color_depth(32);
	  if (set_gfx_mode(GFX_AUTODETECT, 640, 480, 0, 0) != 0) {
	     abort_on_error("Couldn't set a 32 bit color resolution");
	  }

       Note that the screen color depth won't change until the next successful call to set_gfx_mode().

SEE ALSO
       get_color_depth(3alleg4),     set_gfx_mode(3alleg4),	set_color_conversion(3alleg4),	   makecol(3alleg4),	 getr(3alleg4),      desk-
       top_color_depth(3alleg4)

Allegro 							   version 4.4.2					  set_color_depth(3alleg4)