get_color_depth man page
get_color_depth — Returns the current pixel color depth. Allegro game programming library.
Returns the current pixel format. This can be very useful to know in order to write generic functions which select a different code path internally depending on the color depth being used.
Note that the function returns whatever value you may have set previously with set_color_depth(), which can be different from the current color depth of the screen global variable. If you really need to know the color depth of the screen, use bitmap_color_depth().
set_color_depth(3), bitmap_color_depth(3), exrgbhsv(3)