Rust used to have a GC in the past, as well as green threads and a bigger runtime. All of this has been explicitly removed before Rust 1.0. The reasons are well documented in many pull requests and RFCs for the language.

The way the async feature works in Rust is that the asynchronous function is just a syntax sugar that gets desugared into a state machine struct during compilation. The way this state machine works is similar to how one could achieve async in a language like C. It's unfair to dismiss everything as excuses given that the fundamental aim of the language is different.

In Rust async functions are not really colored because again - the async function is just a syntax sugar for a struct you can create and use in a sync context. The colors analogy is only really applicable in a language like JavaScript, where there's no way to poll an async function in a sync context.