User blog comment:IamJakuhoRaikoben/Chapter 424: Avatar, Review./@comment-904888-20150318020939

The way I see it is:

A) His skin turning color:

1. Side effect of Memento Mori. I would not be suprised if that attack had a side effect like this. It was, in Mard Geer's own words, the ultimate curse, and Gray took the full brunt of the attack, so it is entirely plausible that the true damage/side effect of the curse starts to manifest.

2. Side effect of Devil slayer magic. A lot less likely, considering he didn't really 'learn' the magic. Very possible that the nature of Devil slayer magic have corrupting/incompatible effect towards his Ice-Make magic.

3. Something happened during the 6 months prior to his dissappearance, though his expression about the color suggest that he half expected it.

The reason for him joining Avatar might be entirely different:

1. Dark side effect. The corruption effect tainted his thinking.

2. Silver V2: He wants to follow his father's footsteps of destroying dark guilds from the inside, and his skin turning gave him a perfect cover.

Also, anyone got theories as to why Natsu is visiting Sabertooth? My theories are:

1) Sabertooth is closest to their present location and has the highest concentration of powerful mages (Sting, Rogue, Orga, Rufus and Minerva).

2) Fairy tail members: my bet is he is going there to find some FT's for backup (my bet's on Gajeel).

3) Gaining info on corrupting magic, Minerva and Rogue both have first hand experience with it.