In programming the Color Computer, my language of choice is assembly language. It’s the only way to get full performance out of the machine, and to control the hardware directly.
I also highly recommend installing a 6309 if you might be inclined to. They don’t cost much, are fully compatible with the 6809, and give these systems a nice boost in performance and capability if you are writing in assembly.
Here are a few essential references for 6809/6309 assembly. Download links are included below the images for each item.
For beginners, Lance Leventhal’s book on 6809 assembly is a great place to start. It’s a typical ASM manual, and a very decent one.
For Color Computer specific things you’ll need to know, like various hardware addresses, Chris Lomont’s reference is extremely handy. You’ll probably want this close at hand much of the time while coding for the CoCo.
And this one you’ll want handy as well, Darren Atkinson’s excellent guide to the instruction sets for 6×09 CPUs. An absolute must have reference.
Many other reference materials are available of course to help you in your programming efforts, many of which can be found on the Color Computer Archive in the books section. This archive is the principle community repository for all things CoCo. Definitely check it out if you haven’t.