yep, from what i've seen it's random: "When its shell breaks, the core in its center is revealed. You won't know what color will appear until this happens!"
it also has an awesome ability:
"Minior has the new Shields Down Ability, which no other Pokémon has had before. With the Shields Down Ability, it will have excellent defensive capabilities as long as its shell is intact. It will also be protected from status conditions. But when its HP drops below half, its shell will break, and it will change to a form better suited to attacking."