I think its all about the context. If you painted a green square and tried to charge big bucks for it, you’d get no respect and hence no sale. I don’t know the whole story surrounding Kelly’s Green, but I’ll bet it’s rather important to connoisseurs of the work of Kelly, of Kelly’s contemporaries, his predecessors and those whom he later influenced.
Some art is more of a logical and/or emotional construct than it is about aesthetics. A construct can include a momentous pivot point in the history of expression or of perception or a complex commentary or reflection on a culture, etc,. etc. The possibilities for the context really are endless.
A lot of art surely is not meant for “everyone,” and I surely don’t pretend to understand or even like much of it. But my gut and a little inquiry tells me if I should bother to respect it, which I feel is a different matter than judging only its aesthetic.
But Art (with a capital A) certainly can be like a wacky religion that makes perfect sense to those on the inside and appear to be utter nonsense (or heresy, or pornography or garbage) to those on the outside. I’m sure too that Art has its charlatans as much as it has its crowd-pleasing heroes, and its naive one-hit wonders as much as it has its hard working auteurs.
I’m a graphic designer, but no expert on these matters. Your excellent question, one that my wife frequently voices, prompted me to take a stab at it.