Here's my theory: Perfect Zygarde has 5 crystals at its chest, coloured Red, Orange, White, Purple and Blue. Let's just dismiss the white one, and we have Red, Orange, Purple and Blue. Let's think that Red and Blue represent the two Zygarde Cores currently introduced, who have DNA of Yveltal and Xerneas respectively, what about the Orange and Purple ones you ask? They are two new cores which have DNA of the Pokemon Sun and Pokemon Moon mascots respectively, which would all come together nicely to bring harmony to Kalos, with one representing each of the five regions in Kalos, two new ones, Eastern and Southern Kalos, introduced and featured in Pokemon Sun and Moon.