This book is a wonderful reproduction of the very first run of the very first edition of this book.
When "The Emerald City of Oz" was first published, a special metalic green ink was used on the cover, and on the illustrations throughout. However, after the first printing run, it was determined that this ink was far too expensive to continue with, and so a regular green ink was used.
This special edition from "Books Of Wonder" very faithfully reproduces the book as it was originally printed, including the use of a metallic green ink. Other OZ books from books of wonder are similarly faithfull in being full reproductions of the first editions.