Before Wayne Thiebaud began painting pie slices and lollipops in his now iconic Pop pastels, he sharpened his pen in the postwar years as a commercial illustrator. “We often had lipstick ads to make, and you had to draw them quickly, with a kind of advertising Esperanto visual language,” the 98-year-old says brightly, speaking by phone from Sacramento, after a morning round of tennis. For an artist drawn to commonplace objects, lipstick—like gumball machines, another Thiebaud muse—had a candy-colored charm, but it also brimmed with “unlimited potential,” he explains. Uncapped, with the pigment twisted up, the tubes resemble cathedral towers; arranged in single file, as in Lipstick Row (1964), they become soldiers in formation. But in Thiebaud’s vivid lineup, there’s no regimented uniformity (as was the case in 1943, when…