Restaurant style tostones are made using green plantains. Once they begin to turn color, they get sweeter and sweeter. They are fully ripe when they are black.. and I mean black.. no yellow at all. If you make any tostones, make sure to add salt immediately after cooking.. similar to french fries.
When plantains are fully ripe, you peel them carefully(because they are a bit mushy), slice them diagonally, and then fry them once on each side until golden brown. These are called "maduros".
In reality, you can make tostones however you like. If you like a little sweetness to them, by all means, let them turn color and prepare them as you would regular tostones. I personally prefer the authentic recipes.
The next time I make maduros, I'll snap some pictures so you can see just how black they get when ready.
By the way, I am going by Cuban recipes. I don't know which culture was the first to make tostones, but Cuban restaurants make them the best IMO. I may be biased though since I am of Cuban decent.
Texas de Brazil has an interesting twist on cooked bananas. I don't know how they make them, but they taste similar to bananas foster without the ice cream. However, they definitely use bananas and not plantains. I think they roll them in a cinammon and sugar mixture and bake them for only a few minutes. Very tasty!