The funny thing is, even though the suggested algorithm for using Platinum/Colony is as I described, Donald has since revealed that the idea was not necessarily to put them together with Prosperity cards, but simply to see Colony and Platinum about as often as you see Prosperity cards in general. So if you own all the cards, you'd just include Platinum and Colony about 1/8 of the time, regardless of how many Prosperity cards may be in your game.
However, this is one of the few points where I disagree with Donald. From what I can gather, Prosperity was almost exclusively tested using Platinum and Colony, and it shows. I don't think cards like Contraband and Counting House would have made the cut if they were tested mostly in non-Colony games. Likewise, many of the very expensive cards that the set is known for (Expand, Forge, Bank, etc.) are much more worthwhile in a Colony game than a Province game. I have found that pairing Platinum/Colony with more Prosperity cards makes for more enjoyable games. That's just my 2¢.