“Function coloring” is an imaginary issue in the first place. Or rather it's a real phenomenon, but absolutely not limited to async and people don't seem to care about it at all except when talking about async.

Take Rust: you return `Result<T,E>`, you are coloring your function the same way as you are when using `async`. Same for Option. Errors as return values in Go: again, function coloring.

One of your nested function starts taking a "serverUrl" input parameter instead of reading an environment variable: you've colored your function and you now need to color the entire call stack (taking the url parameter themselves).

All of them are exactly as annoying, as you need to rewrite the entire call stack's function signature to accommodate for the change, but somehow people obsess about async in particular as if it was something special.

It's not special, it's just the reflection that something can either be explicit and require changing many function signatures at once when making a change, or be implicit (with threads, exceptions or global variables) which is less work, but less explicit in the code, and often more brittle.