Put oven rack in middle position and preheat oven to 350°F. Line muffin cups with liners.
Whisk together flour, baking powder, and salt in a bowl. Stir together milk and vanilla in a small bowl.
Beat together butter and sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, about 4 minutes.
Add egg and beat until just combined. Reduce speed to low, then add flour and milk mixtures alternately in batches, beginning and ending with flour and mixing until just combined.
Divide batter among muffin cups, filling them two-thirds full, and bake until tops are pale golden and a wooden pick or skewer inserted in centers comes out clean, about 15 minutes. Invert cupcakes onto a rack and cool completely.
For each color, transfer 1/4 cup buttercream to a separate small bowl and tint with food coloring (if using), then frost tops of cupcakes.
*Wilton Paste Food Coloring available at wilton.com.
For best flavor, cupcakes should be eaten the same day they're made.
