In large 1-gallon resealable plastic bag, combine grapefruit juice, olive oil, diced red onion, garlic, rosemary, salt and pepper. Seal bag tightly; shake to mix ingredients. Add lamb; seal tightly and shake to coat lamb with marinade. Refrigerate lamb at least 30 minutes or overnight.  Remove lamb from marinade; discard marinade. Thread lamb cubes, red onion pieces and bell pepper alternately on skewers. Grill skewers over medium heat, cooking until lamb is just done and vegetables are tender. Garnish with fresh herbs, if desired.