I'm not convinced colored functions are a problem, personally. You either color an async function with a Continuation argument and place yields, or you can mark

I'm not convinced colored functions are a problem, personally. You either color an async function with a Continuation argument and place yields, or you can mark it as a CPS function with a keyword, so it's not like coloring goes away by omitting the keyword. And making async pre-emptive introduces a different kind of complexity that can be just as hard, if not harder, to reason about for anything more complex than "park this thread for IO".