You can make it more or less sweet, depending on your taste buds, by adding more or less grapes. The color of the grapes will alter the color of the finished slushee.
You can see below that the recipe calls for one peeled orange, lemon and lime. By peeled I mean that all of the skin and pith is removed with a knife--not peeled by hand. That would leave way too much of the pith on all three of these fruits, and even thought the pith is VERY healthy, that just might make the slushee way too bitter.
Virgin Margarita Slushee
1 large orange, peeled
1 lemon, peeled
1 lime, peeled
red or green grapes, one handful or more if you want more sweetness
2 cups ice
Place all ingredients into a high powered blender and blend until slushee consistency is achieved.