Preheat the oven to 350 F. In a medium bowl, whisk together flour, pudding mix, salt, and baking soda. In a large bowl, mix together butter and sugar until fluffy.
Add in the egg and vanilla extract, and mix until smooth. Add in the flour mixture, and mix until it is fully combined. Add in the food coloring, and mix until it is fully green.
Press the cookie dough into a greased 8-inch square baking dish. Bake for 20-25 minutes.