American had to paint their A300s as well which was part of an anti-corrosion requirement of the manufacturer. They used a grey aluminium-lookalike (almost) colour.
American are unique in having changed their livery just once in the near-80 years since metal aircraft came along in the 1930s, and in fact the only change was from the old orange-flash cheatline to the red/white/blue line around 1970.
In the 1930s all modern aircraft were unpainted, it wasn't until after WW2 that paint first appeared, first as a white top to reflect radiant heat from sunlight, which experiments proved was worthwhile in notable reduction of load on the cabin air con equipment (another positive for paint), and then integrating this with cheatline colours. BOAC were a pioneer of the white-top approach in the late 1940s, to deal with excessive heat on tropical sectors (flight deck crews had been seen operating in their swimwear), firstly just over the flight deck, moving on to the whole upper fuselage,
Only in much more recent times have liveries come along with dark-coloured upper fuselage, a double negative of adding weight by paint without the benefit to the cabin air con of the solar heat reflection. You will invariably find such colour schemes are devised by design houses who have no aviation experience.