There a distinctive difference between color-sorted and color-coded libraries. However, the second may be a subset of the first. But in any case, we'd like to talk about libraries sorted by the colors of their books' covers today.
Most of you probably used Excel or a similar spreadsheet software. If columns are set to be labeled by letters after Z goes AA. And then AB,... AZ, BA,... etc. But when it comes to old books, there were other ways to use letters for sorting.