Bake your favorite cookies according to directions, or use store bought. (let cool)
Use food coloring to tint frosting to desired colors.
Spread frosting on the top of 1 cookie. Press a second cookie, flat side down, on frosting at an angle. Cookies should touch on one side and about 1/2 inch apart on the other, to look like open mouth. Press three mini marshmallows in the frosting for the teeth.
Add additional frosting to the top to attach eyes.