Good question and thanks for the feedback.

It's tough to be 100% consistent with a categorization due to the traditional order of books, but this scheme is generally based on genre rather than individual author. So of the works attributed to John, one is gospel/narrative, three are letters/epistles, and one is apocalyptic/prophetic, hence the three different colors. There are other helpful divisions (undisputed Pauline vs. disputed, major vs. minor prophets, etc.), so maybe it'd be good to offer some options in the color/genre breakdowns.