To chill a martini glass, fill it with ice, then cold water. Let it stand 5 minutes, then drain.
Fill a cocktail shaker halfway with ice. Add tequila, vermouth, grapefruit juice, and bitters. Cover and shake several times.
Stir together silver flakes and sugar on a small plate. Run grapefruit wedge along the rim of a chilled martini glass. Dip rim in sugar mixture to evenly coat. Strain tequila mixture into glass, add grapefruit twist, and serve.