It is a matter of perception and brand portrayal. It's superficial, but that's all that matter at this point, when it comes to displaying cars in photos.
It's subtle changes like color and font usage that can immediately change the perception of a brand.
Again. Marketing and Branding 101.
In real world™ marketing at major MNCs, you make substantive choices backed by facts and data.
Black and silver (and gold in some cultures) are known to be strongly associated with luxury, exclusivity, and upscale goods. There are psychological studies behind this. Can you argue in favor of red or white, especially for an underdog in need of global attention? Sure, but you don't give "umm, car pics are going to be not-black" as an answer to a question about Cadillac's "Americanness."
In this case, it's whimsy masked as managerial action, BS spewed forth because he couldn't answer the question, because he (and Cadillac, and GM, and most of us) have no clue what
American luxury is supposed to be.
"Cadillac is about American luxury!"
'What is American luxury?'
"No more black cars in brochures!"
Reminds me of when Buick's slogan was "The Spirit of American Style" featuring Harley Earl's ghost in TV ads. No one knew what the hell "American style" meant, the message didn't stick, and now Buick's most competent offering is an Opel. I'm all in favor of using nationality to define a brand as Jaguar is distinctly British, Volvo is very Swedish, Lexus is proudly Japanese, and Ferrari is very Italian, but if you can't provide a pithy answer to a basic question about how that nationality is interpreted in the context of your product, you need to find another direction.