As presented, it is not really about going beyond 1 bit. It is beyond 1 color channel, with a twist that the color channels are not independent. And adaptation

As presented, it is not really about going beyond 1 bit. It is beyond 1 color channel, with a twist that the color channels are not independent. And adaptation of dithering algorithms to a black-red-green palette (with a missing yellow) and exploration of their behavior is the scientific content that the article showcases. And the "missing yellow" constraint is important, without it, the problem would be separable and therefore uninteresting.