In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla extract and salt.
Gradually add the flour to the wet ingredients, mixing until a dough forms. Divide the dough in half.
In one bowl, add red food coloring and mix until the dough is evenly colored. In the other bowl, add green food coloring and mix until evenly colored.
Wrap each colored dough in plastic wrap and chill in the refrigerator for at least 2 hours, or preferably overnight.
On a lightly floured surface, roll each dough into a 9×12 inch rectangle.
Place one rectangle of dough on top of the other. Gently press them together.
Starting from one long side, tightly roll the dough into a log.
Wrap the log tightly in plastic wrap and chill in the refrigerator for at least 30 minutes.
Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
Using a sharp knife, slice the log into 1/4-inch thick cookies.
Place the cookies on the prepared baking sheet, spacing them about 1 inch apart.
Bake for 8-10 minutes, or until the edges are lightly golden.
Let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
