Preheat oven to 350F, Line 1 large baking sheet with parchment paper, spray with pan spray. Set aside.
In a large bowl, combine the flour, baking powder, baking soda, and salt. Set aside.
In a separate bowl, whisk together sugar, eggs, and peppermint extract until well combined and smooth. Pour into the bowl with the dry mixture, stir to combine.
Divide dough in 3 even pieces. Take one of the three, divide it in half. Set one of those halves aside - this will be dyed green.
Split the remaining half in 2 even pieces, add one each to the original 2 pieces that had not been further divided. You should have 2 larger pieces and one piece much smaller.
Use green food colouring to tint the smallest piece green, and red food colouring to tint one of the larger pieces red.
Divide each dough up into a bunch of pieces, pile together randomly. Gently squeeze together and knead / fold a few times to marble. Don’t over handle, or the colours will go muddy.
Divide dough in half, form into 2 long, wide loaves on the prepared pan.
Bake for 20-25 minutes, until golden brown on the outside. Remove from oven, turn heat down to 250F.
Allow loaves to cool for 5 minutes.
Cutting on a slight diagonal, slice each loaf into ½" slices.
Arrange slices on the pans. Bake for 15 minutes, flip each piece over and bake for another 15 minutes, or until desired texture.
Remove from oven, allow to fully cool.