Come and get them if you dare!
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You can mix the brown and green tiles. They have the same dome distribution.
What they're for is the Master's set. Characters get +1 to hit if they use a weapon from their age and are standing in a hex of their age.
Green keys are the Ancient keys.
Brown tiles are Colonial.
Blue tiles are Modern.
Purple tiles are Future.
All the platters in the Basic Set are Ancient. The Colonial, Modern, and Future platters are in the Master's set.
What Adam said. During platter selection a team can choose whatever platter they wish. In a game I am currently playing solo one of the teams has no ancient characters so naturally I chose no ancient platters or keys for this team during placement. No point in choosing that terrain if that team will not be able to receive hit bonuses.