Almost all the white plastic covering of air conditioners seem to turn yellow here in the tropics. I read an interesting article on how to get the plastics white again and it also mentioned spraying the plastic with a clear acrylic varnish to prevent the plastic from getting yellow again.

Is this a good idea ?

Spray with acrylic white paint.

a better idea would be to paint them all black, or blue or red,(enamel) or......any other colour other than white.

the problem is the in the plastic and all plastic will discolour in sunlight but mainly its ultra violet light that reacts with it.....fluorescent lights for example and of course the usual "beyond the spectrum"

not sure where your acryllic idea comes from; but a black bin liner would be just as effective; but of course thats silly.