I may be totally wrong with this assumption, but the way I understoog Zig's color-less async support is that the compiler either creates a "red" or "blue" function body from the same source code based on how the function is called (so on the language level, function coloring doesn't matter, but it does in compiler output).

The compiler still needs to stamp out colored function bodies because the generated code for a function with async support needs to look different - the compiler needs to turn the code into a state machine instead of a simple sequence).

It's a bit unfortunate that red and blue functions appear to have a different "ABI signature", but I guess that's needed to pass an additional context pointer into a function with async support (which would otherwise be the implicit stack pointer).