1. Preheat oven to 350°.
2. To make cupcakes, combine flour, baking powder, and salt in a medium bowl.
3. Beat butter and sugar with an electric mixer on medium speed until light and fluffy. Add eggs; mix until smooth. Add half of flour mixture, and combine; repeat with remaining flour mixture. Add yogurt and vanilla; blend at low speed. Blend in zest. Scoop dough into 48 mini baking cups (about 2/ 3 full); bake until tops spring back when touched lightly in center (about 10 minutes). Remove from oven; cool completely.
4. To make frosting, place cream cheese in a medium bowl; beat with an electric mixer on medium speed until smooth and creamy. Add powdered sugar, vanilla, grapefruit zest, and grapefruit juice; beat until smooth.
5. Divide the frosting into 2 bowls. Add food coloring to 1 bowl; mix with a spatula until frosting is light pink. Frost cupcakes according to directions in "Ooh! Easy Frosting Trick" (below).
Ooh! Easy Frosting Trick
To frost minis, place a dollop of frosting on top, and swirl with a spatula like a Mini Offset Stainless-Steel Spatula ($16; williams-sonoma.com). Put frosting in a piping bag with a large round tip (or use a zip-top plastic bag with a corner cut off), and pipe a large dollop on top of each cupcake. Then use another bag with a smaller tip and a different color frosting to make a "bull's-eye" on top of each dollop.