Green means go, red means stop. Trophies or confetti come with good performance, and people who fall behind get nudged to do better. Those brightly colored engagement tactics long ago jumped from smartphone games to everything from online shopping to sports betting and classrooms. So it should come as no surprise that many nutrition-tracking apps like MyFitnessPal and Noom also use gaming features to keep users coming back. But as nutrition apps proliferate, some researchers are raising alarms...