In a mixing bowl or kitchen mixer, beat softened butter for about a 1 minute on medium speed. Add the sugar and vanilla extract and beat for another minute until fluffy.
Slowly add your baking powder, salt and flour into your mixing bowl on low speed. Beating until combined, about a minute. Add the milk and beat until just combined.
Divide the dough into two equal parts in two seperate bowls. Add a few drops of blue into one of the bowls and the green into the other bowl and stir until the dough is the color you want. Form into a ball and wrap each in plastic wrap and refrigerate for 1 hour.
After the hour, take your dough out and work quickly as you want the dough to stay as cold as possible. Take about 1 teaspoon of each color and mix together in your hand, roll into a ball and smash each ball onto an ungreased cookie sheet.
Bake cookies at 350 degrees for 10-12 minutes or until the edges turn a slight golden brown. Allow to cool on baking sheet and then transfer to a wire rack. Shortbread cookies are best when eaten completely cooled.