Green threads is very different from CPS actually both from an implementation and semantics point of view. In a CPS model the continuation is first class and is available at every single call point. In practice this would require heap allocating every single function frame, which is expensive and and a huge problem for interoperability (a smart compiler can turn it back to a traditional stack if the continuation is only used in traditional ways)

With the normall call/return model, the implicit continuation in a normal call stack can only be accessed via return. Green threads do allow accessing the continuation at specific yield points (although most green thread implementations do not expose it to the user) but because the yield point are much fewer than every single call, a whole stack can be allocated and used in one go. There aren't in fact many issues with interoperability as foregin functions or even os calls can be accommodated on this stack.

Async is a tradeoff, basically the programmer is responsible of marking functions that are to be CPS transformed and that need their stack frame reified. This way no full stack is needed nor there are the performance and interoperability issues of full CPS. You end up with the blue/red functions issue though.