Nice article and good job implementing it. I would have preferred a small explanation of the filter graph option (-lavfi), but I guess that's outside the scope of the post.
Yeah, but the article is already quite long so I omit the "details" on that...
Basically, it's just to specify a complex filtergraph: in the case of -vf you're just passing one video stream which gets filtered by successive filters. In the case of -lavfi (or -filter_complex if you prefer) you can feed multiple inputs (and get multiple outputs). In this case, the second input is the palette image.
