Heat oven to 350°F. Spray 13x9-inch pan with cooking spray. Make and bake cake mix as directed on box for 13x9-inch pan, using water, oil and egg whites and adding pink paste food color. Cool completely.
Line cookie sheet with waxed paper. Crumble cake into large bowl. Add frosting and chocolate chips; mix well. Shape into 32 oblong balls; place on cookie sheet. Freeze until firm. When cake balls are firm, transfer to refrigerator.
Remove several cake balls from refrigerator at a time. Dip tip of 1 lollipop stick 1/2 inch into melted white candy and insert stick into 1 cake ball no more than halfway. Dip each cake ball into melted candy to cover; tap off excess. Poke opposite end of stick into foam block. Let stand until set. Dip each cake ball into melted green candy to cover; tap off excess. Return sticks to foam block. Let stand until set. With toothpick, decorate cake balls with light green candy to look like watermelons. Let stand until set.