The first thing that you should ensure to have success in the whole process is to take out all the books from the shelves and arrange them to suit their color description. If you are considering an ombre effect, you will want to sort your books within color groups from dark-to-light shading and vice versa, and then this is a trick that can be employed.  There may also be many opportunities for creativity if one wants something more specific such as different shades of blue or white covers. These additional options could help add interest on top of whatever theme you already had gone with just colors alone. And with these, nothing will be more clean and elegant than your mid-century bookcase.