Preheat the oven to 350 F. 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. Divide the batter into two bowls.
Add green food coloring to one bowl and blue to the other. Pour each batter into a greased 8-inch square baking dish, and swirl with a knife. Bake for 20-25 minutes.