Preheat the oven to 350°
Blend the softened butter, the 2 cups of flour and the 1/2 cup of sugar in a medium bowl.
Press evenly in the bottom of an ungreased 9" x 13" pan
Bake for 15 to 20 minutes in the preheated oven. When done it should be firm and golden.
In a separate bowl, whisk the flour and sugar
Whisk in the eggs and the lemon or lime juice. Add the food coloring if desired.
Pour the filling over the baked crust.
Bake for an additional 20 minutes.
Let the bars cool out of the oven before cutting. They will firm up as they cool.
Prepare one batch each of the lemon and the lime bars. Cut them all into squares and arrange them in a checkerboard pattern for an impressive dessert.