A car stopped at a red light, next to a line of people that snaked around the block. The kid in the passenger seat rolled down the window and shouted the obvious question: “What’s this for?” He got a quick reply, if not a full explanation: “It’s the mustard Skittles, man!” This week, to promote its new, limited-edition flavor—a collaboration with French’s mustard—Skittles turned candy into an event in downtown Washington, D.C. Hashtags, Instagram-ready backdrops, a cornhole-like game named Tang Toss, a general air of manufactured whimsy: It was all there. Some people walking by gazed upon the scene with expressions of curiosity and confusion and pity. Some, shrugging, joined the line. I was one of the shrugger-joiners. (I happened upon the, uh, Mustard Mobile—a repurposed Volkswagen van—and my curiosity got the better of me.) As I waited, the queue grew behind me, looping around a wide city square, as onlookers took photos and videos and replies of “mustard Skittles!” occasionally punctuated the downtown din. The event was a classic marketing stunt, right down to …