Man pages matching name gui_bg_color in section 3alleg4:

gui_bg_color.3alleg4.en	debian 10 / allegro4-doc
gui_bg_color.3alleg4.en	debian 11 / allegro4-doc
gui_bg_color.3alleg4.en	debian 12 / allegro4-doc
gui_bg_color.3alleg4.en	debian 13 / allegro4-doc