What Enel did was turn into lighting itself, and increase the size of the said lighting. However, what Amon did was let lighting flow through his body. Those two things are different, so Amon's skin didn't turn blue completely, only his hair did, which mixed with the air of the forest where strange plants live, turning it green.
One Piece: Reborn as a Skypiean
Anime & Comics · Master4thWall
detail