Thanks for the answer. Dsell, can you quote the "colonies and platinum must be played at the same time" rule? We have Intrigue and Original Dominion and couldn't find the rule there. Did we overlook something?
Here's the rule:
"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."
From the Prosperity rulebook.