I'm in the process of rewriting the CoCo driver, and my intention is to rely on HLSL for artifacting, rather than approximating the result the way the current driver does.
It is my understanding that HLSL expects output more faithful to NTSC timing than what the CoCo driver currently outputs. Is there an example of a driver (in either MESS or MAME) that does this correctly?
I can't answer your question but wish you the best of luck in your efforts.
If you don't have actual CoCo hardware to observe, you need to know that for the most part, the only time artifact colors are seen on a CoCo is when the color burst signal is turned off. That happens in PMODE4 but no other PMODEs. You can get artifact colors in unsanctioned HSCREEN modes but there is no hardware information on which to base MESS emulation. Currently the HSLS software with yiq enabled does not make this distinction. The result is that all other CoCo text and graphics screens are the wrong colors.
The CoCo3 in 40 or 80 column text does cause problems for TVs or CMP monitors but that is because they cannot cope with the high resolution. This is not artifacting in the usual sense.
Not yet as far as I know, but if you'd like, I can update the Apple II driver to do so (it has the simplest video driver that I can update to output "NTSC-shaped") pixels. Put that on my to-do list in the other thread, please.