1Preheat oven to 350°F. Empty brownie mix into large bowl. Add sour cream, eggs and red food color; mix well. Spoon batter into greased foil-lined 9x5-inch loaf pan.
2Bake 55 to 60 minutes or until toothpick inserted into center comes out with fudgy crumbs. Cool brownie in pan 10 minutes. Remove from pan; cool completely on wire rack.
3Gently stir whipped topping, lemon extract, 1 teaspoon neon blue and 5 drops neon purple food colors with spatula until evenly tinted. Stir in 3/4 cup of the marshmallows. Set aside.
4Line loaf pan with foil with ends of foil extending over sides of pan. Cut brownie in half horizontally into 2 layers. Place bottom brownie layer in loaf pan. Gently spread ice cream over brownie layer. Place top brownie layer over ice cream. Spread whipped topping mixture over top. Press remaining 1/4 cup marshmallows into whipped topping. Cover carefully with foil.
5Freeze at least 2 hours or until firm. Use foil handles to remove dessert from pan. Place on cutting board; let stand 10 minutes to soften. Cut into slices to serve.