Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish or line it with parchment paper.
In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.
Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the red and blue M&M candies and white chocolate chips.
Spread the cookie dough evenly into the prepared baking dish.
Bake for 25-30 minutes, or until the edges are golden and a toothpick inserted in the center comes out clean.
Allow to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.
Once cooled, cut into bars and serve.