If desired, rub rims of margarita glasses with lime wedge; dip rims into a shallow dish of coarse salt or sugar and shake off excess. Set aside.
In a blender, combine limeade concentrate, orange juice, and grapefruit juice. Cover and blend until smooth. With the blender running, gradually add ice cubes through the hole in the lid, blending until slushy. If desired, tint with a few drops of green food coloring. Pour into margarita glasses. If desired, garnish with citrus slices. Makes 8 (4-ounce) servings.