At one time in my life I really enjoyed Xanth books. It was around middle school to early High School when I mostly stopped reading them altogether. The damage was done. The puns and sexual innuendo of the fantasy realm enchanted my young mind. Being that the majority of the Xanth novels started in the 70's and 80's, cover variations were pretty unique, and sometimes not-so. Exhibit A: Ogre, Ogre.
The color of the books is not a product of fading, it's just what happens between printings. Typically the covers get lighter as the printings run on. Only one of the above is a first edition (I'll let you decide). Typically there will be a blurb of some sort over the cover art saying, "A Xanth Novel", or "A New Xanth Novel!", or the like. I hate the blurb like I hate herpes. It just ruins the great art on some of these older books.
I just realized how boring this post is so i will quickly end it with this: Collecting variations of book printings is nearly impossible and has no real value. What is written inside is still the same, you wont be missing anything, and really its just tough to do. My advice is to just get the first edition if you want it and always keep your favorite books that you read the first time. If you can't find the first edition, use that money to buy beer.. or herpes medication.