Catan cities and knights has four colored circular tokens. There is a yellow, blue, and green to indicate when someone has acquired the metropolis for trade, politics and science respectively. There is also a white and grey one. My question is what is the white and grey one for? I couldn't find any reference to it in the rules.
It's a spare piece. Usually these kinds of tokens are printed on a sheet of cardboard that's big enough to fit in the game box, and if there's a bit of extra space then a few extra things might be put on the board to fill the space. Those tokens can then be used as replacements for the main tokens - so if, for example, you lost the green one, you can just state that the grey token replaces the green.