Cook sugar in a small saucepan over medium heat, tilting pan occasionally, 10 minutes or until caramel colored. Remove from heat, and gradually pour white grape juice over sugar (mixture will bubble and seize). Let stand 5 minutes.
Cook mixture over medium-low heat, stirring occasionally, 15 minutes or until sugar is dissolved (mixture will be syrupy). Remove from heat, and let cool 30 minutes.
Combine grapefruit and orange sections in a bowl. Pour juice mixture over fruit. Cover and chill 2 to 24 hours. Garnish, if desired.