Re: rules for variable city tiles?
We use them on all the maps. Start with a Red one on each red City.
When players want to Urbanize for $10k or use the Operations Card allowing a city upgrade, apply a rotor of choice to a Grey City. Rotors start on their solid color, the color wheel portion rotate counter-clockwise.
(We hand out the Cheat Cards from Western US, as it has the color wheel in top right corner, allowing planning ahead)!