Usually, when Washington, D.C., commuters are inundated with mint green-tinted ads in March, it means the Shamrock Shake is back at McDonald’s. This year, the eye-catching color instead appears in a full-court-press ad campaign for prediction market platform, Kalshi, which uses that shade in all its branding. Unlike seasonal milkshake ads, though, the targeted barrage of billboards, bus stop signs and metro station posters isn’t meant to reach all residents within the nation’s capital;...