Using that old mac, I was able to get shiftpwm running on my uno. I have 2 test setups both use 8 RGB LEDs an 3 Shift registers. Each setup has different pin grouping that I havnt figured out yet. One setup uses 1 chip for each color (red, green, blue), the other one is setup with the LEDs interlacd RBGRGB....
The one that is RGBRGB seems to work as expected, but the other one seems work wrong, even if i set its pingrouping to 8.