I think it's here:
Actions. The player with the current
prestige advantage begins the first half
decade. He chooses 1 card from the
card display or 1 card from his hand
and performs 1 action.
6 THE ACTIONS
The action cards are the “engine” of the
game. A normal card may be played in 1 out
of 4 ways. You may choose to:
• remove 1 unrest counter, or
• build up your economy, or
• increase your living standard, or
• trigger the card event.
6.1 Remove 1 unrest counter
With any normal card, you can remove 1 of
your unrest counters.
6.2 Build up your economy
You can use any normal card for this action.
6.3 Increase your living standard
You can use any normal card to place up to
3 living standard counters
6.4 Trigger the card event
East may only trigger Red and Dual Card
events. West may only trigger Yellow and
Dual Card events.
I.e. 3.3 says "chooses 1 card", and only 6.4 (event) mentions any restriction on card color.