Are we...seriously trying to debunk pokemon based on their color palette, folks? If Dratini came out in Kalos, and Dragionite were to be leaked, what then? I'd imagine the result would be mass hysteria. I like Greninja, but don't confuse this post for someone who's set on believing it's real yet-- I just find that trying to debunk anything on something so minor is quite odd.
I see why trying to size Greninja up against past starters' colors, but I think we can agree that trying to find patterns in GF's logic today is pretty baseless: Pokemon Black/White2 proved that, and XY set out to prove that even further. Not to shut down those of you that support judgment using that logic, but the same thing can be argued against with Ninjask/Shedinja, who not only carries no matching color scheme to Nincada but don't even remotely look like it.
As much as I like him, the previous false leak of Froakie3 honestly looked more convincing than Greninja--and, using the logic of color palettes, he should have been Froakie's true final evolution. Whomever made him clearly designed him using this same mindset to make him feel more natural, and (SURPRIZE) it ended up being fake.
I feel as though there are better ways to argue against Greninja's validity than to judge his coloring.