And have courts ruled that cities (or other governments) can't impose color restrictions?
>yard sign restrictions, political activism restrictions
Not sure what's meant by "political activism restrictions", but cities most definitely have restrictions on signs. You can't put a brightly flashing LED sign next to a busy intersection, or plant a billboard to make some money on the side.
>and free association (visitor restrictions).
???
I've literally never encountered this, unless you count having to register to use the visitor parking.