According to Alex Katz, speaking publicly at London’s National Portrait Gallery on Friday with Nation art critic Barry Schwabsky and the Gallery’s director, Sandy Nairne, Americans size up someone immediately by their clothes and their haircut. (For Brits, by contrast, it is accent that determines class, and for the French, sentence structure.) He insists his portraits, like all his work, contains all the information in its surface, and that he has no interest in psychology.

For his commissioned portrait of American Vogue editor and “September Issue” star Anna Wintour, however, Mr. Katz succeeded in coaxing his sitter to shed her trademark layer of social armor, her sunglasses, to reveal sparkling eyes that accent and animate her otherwise meticulously serene visage. Her bob cut catches light to define volume that defies the obstinate flatteneing of the rest of the image. The tour de force of the painting is its marigold ground—it is a deceptively bright, glowing field of color because it is actually darker than the fashion icon’s demur skintone and restrained jacket.

A small display accompanies the presentation of this new portrait, until September 21, which includes Mr. Katz’s masterful and innovative group portrait in cutout aluminum, One Flight Up, from 1968.

Mr. Cohen, editor of artcritical.com, is art critic of the Sun.