Dominion: When to use Platinum and Colony
From the Prosperity rulebook:

If only Kingdom cards from Prosperity are being used this game, then the Platinum and Colony piles are added to the Basic cards in the Supply for the game. If a mix of Kingdom cards from Prosperity and other sets are being used, then the inclusion of Platinum and Colony in the Supply should be determined randomly, based on the proportion of Prosperity and non-Prosperity cards in use. For example, choose a random Kingdom card being used - such as the first card dealt out from the Randomizer deck - and if it is from Prosperity, add Platinum and Colony to the Supply. Platinum and Colony are not Kingdom cards; when those are included, there are 10 Kingdom cards, plus Copper, Silver, Gold, Platinum, Estate, Duchy, Province, Colony, and Curse, in the Supply. Use 8 Colonies for a 2-player game, or 12 Colonies for a game with 3 or more players.

Seems like there isn't much I can add to that!