If You’re The Deepweb Assassin It’s About Damn Ti

Okay so at the end of the show it's revealed that green is the imposter but I don't understand how this makes sense and seems to just condradict several earlier plot points in the show:

Green claims to have killed everyone except Lime. But how did they kill Cyan when they were with Brown and Purple?

They apprantly killed Brown as well despite clearly being shown in the cafetria with everyone else

Purple initially only works out that Green was the imposter because Green didn't remember the story Purple told them in Oxygen. But if Green was the imposter the whole time wouldn't they have just be present for the story anyway?

The only explanation that makes sense is that Green was turned into the imposter at some point after being in Oxygen. But doesn't that contradict Green saying they killed everyone? Also the doctors scan revealed they were an imposter so they must have been one the whole time?