1Heat oven to 350°F. Line 13x9-inch pan with foil so foil extends about 2 inches over short sides of pan. Grease bottom only of foil with cooking spray or shortening.
2Make and bake brownie mix as directed on box for 13x9-inch pan, using water, oil and eggs. Cool completely, about 1 hour. Remove brownie from pan by lifting foil; peel foil away. To cut brownie into triangles, cut lengthwise into 3 rows. Cut each row into 7 triangles, see diagram. Save smaller pieces for snacking.
3Stir food color into frosting. Spoon frosting into small resealable food-storage plastic bag; partially seal bag. Cut off tiny bottom corner of bag. Squeeze bag to pipe frosting over brownies. Sprinkle with decors.
4Break off curved end of candy cane; insert straight piece into bottoms of triangles to make tree trunks.