I contacted a former coco programmer letting him know about this artifact mode and the possibilities it has. Mr Dekok ended emailing me back saying it was about time people looked at this quirk in the Gime chip, As he played around with it back in the late 80's or early 90's but never did anything with it as back then most users were using RGB displays.
Nice to see you here, too, briza - I hope you joined because of Arduino, and not just this post! One of these days, I plan on hooking my Arduino up to Roger Taylor's CoCo3.com RS-232 pak (I got the "Hacker's Special" a while back, but have yet to put it together - it came unassembled, without a cartridge or the bluetooth module; the RS-232 output though, IIRC, is TTL - so it should be able to communicate with an Arduino easily).
The thing about the CoCo 3's "high color" artifacting mode is that in a way, it was discovered. There was an article in the Rainbow about how to use it; that's what I played with. But it was published as a means to get a few extra colors on the high-res (640x192) screen, which could only officially use four colors from the palette of 64 - the article detailed how you could get several more, as long as you used a composite monitor or TV. From playing with it, you could only get so many extra colors, something like 8 or 9 dithered colors.
No one - except perhaps for your programmer friend - ever thought to use the four grey scales available on the CoCo 3 in that mode, dithering them, to cause artifact colors. I must say it seems tragic that your friend didn't attempt to publish the method to the wider public; even a simple letter to the editor of the Rainbow would have been sufficient. Instead of it laying dormant, perhaps those games and other applications that could've taken advantage of it would have been written. History is history though, unfortunately.
I am still uncertain, though, whether this method is the fabled "256 color mode" Sockmaster and others have been pursuing off-and-on for years; it doesn't seem to match up with what has been discussed and noted (no strange twiddling of the GIME registers or such). Even so, it was definitely a very obscure mode. I am hoping Roger Taylor takes advantage of it with a new game!
