JW the path from author to user. The guidelines specify access requirements that can be met along the processing chain. Semantics and structure could be introduced as well. We could abandon the need for alt-text if UAs had optical character recognition. Yes, there are LK also what we are assuming UAs don't do.
Color is a newer technology.
It feels like it fits somewhere under guideline 1. It could also go uder 2. If the author provides color, they should also provide appropriate contrast. Likely to be included in the Introduction. Perhaps under 1.5 - benefit of doing this.