Combine the flour, baking powder, and salt in a large mixing bowl; set aside. Combine the butter and shortening in the bowl of an electric mixer and cream until smooth. Slowly add the sugar, beating on medium. Add the eggs, one at a time, beating well after each addition. Add the vanilla. With a wooden spoon, stir the dry ingredients into the creamed mixture 1 cup at a time. Turn the dough onto a floured surface and divide in half. Place each half on a large sheet of plastic wrap. Flatten into disks about 1 inch thick. Wrap separately, then refrigerate at least 2 hours or up to 1 week. To bake, preheat oven to 350° F. Roll to 1/8 inch thick on a well-floured surface. Cut into shapes. Sprinkle with the colored sugar, if desired. Using a spatula, transfer to a large greased baking sheet, sugared-side up. Bake 12 to 14 minutes or until light golden brown around the edges. Transfer to a cooling rack.