> Function coloring is specifically about requiring syntax for a function, eg. the async keyword.

Someone should tell the inventor of the phrase, because they don't mention the async keyword at all[1]. As-written, function coloring is about callbacks (since that's semantic mechanism that JavaScript happens to pick for their asynchronous model).

Function coloring is just an informal way to describe encoding a function's effect. You can encode that in syntax if you want (an `async` keyword), or in the type system (returning `() -> T` instead of `T`), or in the runtime itself (by controlling all I/O and treating it the same). But you can't avoid it.

[1]: https://journal.stuffwithstuff.com/2015/02/01/what-color-is-...

They specifically called it out as a syntactical issue, where the issue was based around the requirement to have the 'red' or 'blue' keyword. The section on "2. The way you call a function depends on its color." makes this pretty explicit...

    2. The way you call a function depends on its color.

    Imagine a “blue call” syntax and a “red call” syntax. Something like:

    doSomethingAzure()blue;
    doSomethingCarnelian()red;

    When calling a function, you need to use the call that corresponds to its color.