Our annual holiday mailing went out last month. Pick up a copy of it. It includes a 5th coaster for Martian Coasters.
Martian Coasters is a Treehouse game in which each player is assigned a gameboard of a particular color. The set includes 4 colors: Red, Yellow, Blue, and Green. Thus, only 4 can play, even though the Treehouse set (which the game requires) comes with 5 colors.
This problem-statement is presented in cartoon form on the front of the 2006 Holiday Card. (You can see those cartoons by going to the WWN article for December 14, 2006). Inside the card is a free copy of the thing you need to make that fifth color work: a Black coaster!