Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, combine butter with sugar until fluffy. Add in egg and vanilla extract, and mix until smooth.
Add in the flour mixture, mix until it is fully combined. Remove half of the cookie dough into a separate bowl. Color one half of the dough blue with the blue food coloring, and the other half green with the green food coloring.
Take a small amount of both the green and blue cookie dough, and roll them together into a ball. Place the cookies onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.
