There is only a List custom metadata tool, not one that has the capacity to store an associated colour along with a text label. There is a feature request for such a thing already.
By the way, I wouldn’t underestimate the Keyword tool as a secondary colour-coding tool. Unlike a hypothetical custom metadata field type, which wouldn’t feasibly have any display in the user interface outside of the Inspector and Outliner, keywords can be displayed in the Corkboard too, as well as the Outliner. Marking the “difficulty” for a section strikes me as a good use case for keywords.