Apparently Americans are on the must self-quarantine list at the moment, but not if they've spent the past 14 days in a "green" country.
THE big problem is that all foreigners, whether coming from green or red countries, must present an insurance certificate in acceptable form covering all COVID expenses for 14 days after arrival. So far as I can gather, there is no such insurance available, there is no such certificate available. Only resolution would be if some state insuror starts offering the insurance at borders/airports. I've come across no sign of that yet.
(Of course it's also conceivable the insurance requirement in practice is being winked at, as some previous Ukraine insurance requirements were often winked at. Not that anyone in their right mind would fly to Kiev just to find out.)