As mentioned elsewhere, there are some CoCo-related extensions available for the Microsoft Visual Studio Code editor. I did not realize that MS released this editor for Mac and Linux as well as Windows.
Here is the free editor:
Once installed, you can download (from within the editor) extensions.
Here are two extensions by Tandy UK — one for COLOUR BASIC and the other for 6309/6809 assembly:
There is another 6809 assembly extension available:
It colorizes files ending in .a or .asm.
I don’t see many details about the Tandy UK ones, but I am indeed getting colorized BASIC keywords when I have a .bas file.