Last time I was buying a pineapple at Costco, a nice older Asian lady gave me some advice. She told me that I should choose a pineapple that was uniform in colour (most will be yellower at the bottom and greener at the top). She said that will help it ripen more evenly and be sweeter. And the best way to ripen a pineapple? Put it into a plastic produce bag, tie the top (I don't think you want it completely sealed shut), and wait a couple of days.
I figure, if someone stops you from buying your (non-uniform in colour) pineapple, takes your pineapple out of your hands, passes you a new one with a plastic bag to put it in, they must know what they are talking about. And do you know what? It totally worked. It was an awesome pineapple! Try it!