Man pages matching name polytype_grgb in section 3alleg4:

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