Bake each cake in a 10-inch fluted tube pan according to package directions. Cool cake in pan for 15 minutes. Remove cake from pan and cool completely on a wire rack.
Combine frosting and powdered sugar. Use food coloring to color frosting. Place one cake, rounded side down, on serving platter. Spread a layer of frosting over top. Add second cake, rounded side up, to create pumpkin shape. Be sure to line up the ridges of the cakes.
Fill the center well with trail mix to 1 inch from the top. Frost top and sides of the cake, leaving an indent at the top. Insert a wafer cookie into the top of the cake for a stem. Cut remaining cookie half diagonally. Place one half of the cookie, cut side up, on front of cake for a nose. Decorate the face as desired, using gel icing and assorted candies. If desired, decorate both sides of cake with contrasting faces.