Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, combine butter and sugar until fluffy. Add in egg and vanilla extract and mix until smooth. Add in the flour mixture, and mix until fully combined.
Divide the cookie dough into three bowls, and add purple food coloring to one, yellow to another, and green to another. In a shallow dish, combine granulated sugar and ground cinnamon.
Take a small amount of each cookie dough and roll together into balls and roll into the cinnamon sugar mixture. Place cookies onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.
