Who decides which metropolis to steal?

Question

Say the red player has two metropolises (having upgraded both politics and trade to level 4). Now the blue player upgrades their politics to level 5 and steals a metropolis from red.

Who decides which of red's two metropolises will be given to blue? The significance being that the city losing its metropolis is now susceptible to sacking by the barbarians.

I've avoided such ambiguity by throwing out the metropolis tokens and painting the metropolises yellow, green, and blue. But that's just a house rule.

Answer 1

It is not explicitly stated in the rules.

A large Catan faq (German only) argues that, because the rules don't distinguish between metropoles from different branches, the player who owns them may decide which one to revert.

What you describe is a common house rule (I use it as well). In the faq it is mentioned that some people paint the metropoles to make this house rule more visible.