One rainbow cocktail, coming right up! ...Except I generally use orange juice instead of pineapple juice because 1.) it's more common and 2.) I don't really like pineapple juice :|
I /think/ grenandine mixed with orange juice should be denser than pineapple juice, which I could layer on top of that. And if I mixed blue food coloring with soda, or just use blue-raspberry-flavored ice, that would top it off.