Check the publisher’s imprint on the spine or title page.
Books with copyright dates before 1919, but published by Reilly & Lee, are actually later printings.
The photo shows typical spine imprints for Reilly & Britton and Reilly & Lee.
Reilly & Lee did publish first editions of some Oz books by other authors than Baum.
I will probably ask you to send photos to assess their edition and condition.
I also purchase early Oz, Baum, Denslow and Roycroft ephemera (paper items).
Conversely, just because your book color plates doesn’t mean it’s a first edition.Note that Oz books have a copyright date on the back of the title page, but not the actual printing date.So, if your Oz book has a pre-1936 copyright date, but has no color illustrations inside, it is a later printing.It just means it was printed before approximately 1935.Use the reference books given above, or take your book to a knowledgeable bookseller to tell for sure.
(The exception to this is the first edition/printing of The Road to Oz, which was printed on different colored pages and never had actual color illustrations.