Put yourself in her shoes. If she derives joy from a colorful sorting of the books, it’s likely any other sorting would cause some annoyance/anguish (however minor). She would feel this way each time she laid eyes on the bookshelf.
On the other hand, a colorful sorting would only negatively affect you when you are actively looking for a book. I’m guessing that’s far less often than she merely looks at the bookshelf.
A compromise could be that she sorts the books by color and then by authors name. Not the most efficient sorting method for finding books, but would save a considerable amount of time compared to no author name sorting at all.
Edit: alternatively, a quick Google image search of the book name would tell you all you need to know to quickly find it in a colorful sorting.
I know, right. It’s crazy to me that people need to actually read the spine of a book they own in order to know what it is. Like, don’t you know exactly what it looks like? It’s your book. In a personal library, it makes sense to group books by how similar they look.