The event Zygarde cannot be separated. Why? It's because of its ability, power construct. If you try to separate the event Zygarde, the game will say: "The cells are bound together too tightly! It can't be separated!", which would be the same for any other complete Zygarde. But, if you had a shiny Zygarde without power construct, then you could separate it and lose the shininess.
The shiny 100% Forme has the same colors as shiny 50% Zygarde.
Source: I just tried this in Ultra Moon