Place marshmallows and butter in a microwave-safe bowl; microwave, uncovered, on high until butter is melted and marshmallows are puffed, about 45 seconds. Tint with green food coloring. Stir in cornflakes. , On a waxed paper-lined baking sheet, divide mixture into 8 portions. With buttered hands, working quickly, shape each portion into a 3-in. wreath. Decorate immediately with M&M's, pressing to adhere. Let stand until set.