Preheat the oven to 350 F. Line a 12 cup muffin pan with paper liners. In a large bowl, mix together butter, brown sugar, eggs, and vanilla extract until smooth. Add in the flour, salt, and baking powder until smooth.
Evenly divide the batter between two bowls. Mix blue food coloring into one bowl and green into the other. Evenly divide both batters into the paper liners. Swirl with a knife. Bake for 12-15 minutes.