Absolutely, there can be a huge difference, and Trade Dress is a sub-category of trademark, not copyright. It has to do with product identification; the cover design of the For Dummies books is a classic example, the distinctive Louis Vuitton pattern is another. Just google "trade dress" and skim through the various definitions.
At least on the surface, this doesn't come close to being trade dress (and in my business, I do apply both trademark and trade dress, so I'm not totally unfamiliar with the distinctions). How could this confuse a consumer into buying the wrong computer, or even hiring the wrong artist? The answer to the question, "Who did the Apple ads, I want to hire him?" is not, "Britto." Might there be some subtleties that we're missing? Of course. The law often turns on the small details.
If it's appropriate to sue the artists, it's completely appropriate to sue the artists' customer. If in fact there is a trade dress violation, Apple has an obligation to be aware of it - it's Apple that made the work public. That's why publishers are sued when writers plagiarize, "He didn't tell me he plagiarized" is not a valid defense.
An artist most definitely has a style, and that style is undoubtedly derivative of the artists who came before. A key factor in making a career in the arts is finding a way to distinguish ones self from every other person who studied at the same schools, went to the same museums, and listened to the same performances.
While imitation is supposed to be the sincerest form of flattery, I can see someone like Britto being ticked off if another artist got paid big bucks for work so similar in style to his own. To adapt a joke that's gone around in creative circles for a long time...
The three stages of a career: "Who's Britto?" "Get me Britto." "Get me a young Britto."
