You can exit an async/IO monad just like you can exit an error monad: you have a thread blocking run(task) that actually executes everything until the future resolves. Some runtimes have separate blocking threadpools so you don't stall other tasks.

If you have something in a specific language that does not result in having to change the entire call stack to match something about it, then you do not have a color. Sync/async isn't a "color" in all languages. After all, it isn't in thread-based languages or programs anyhow.

Threading methodology is unrelated though. How exactly the call stack is scheduled is orthogonal to the question of whether or not making a call to a particular function results in type changes being forced on all function in the entire stack.

There may also be cases where you can take "async" code and run it entirely out of the context of any sort of sceduler, where it can simply be turned into the obvious sync code. While that does decolor the resulting call (or, if you prefer, recolor it back into the "sync" color) it doesn't mean that async is not generally a color in code where that is not an option. Solving concurrency by simply turning it off certainly has a time and place (e.g., a shell script may be perfectly happen to run "async" code completely synchronously because it may be able to guarantee nothing will ever happen concurrently), but that doesn't make the coloration problem go away when that is not an option.