Since it's on the cover, I might as well do them first.

Before:

I think this is what they were trying to get at in the games, but the normal and shiny look too similar. (I think this palette would suffice as a shiny, but it still looks similar to regular Garchomp.)

I actually don't mind Mega Garchomp's purple and pink color combination... it just isn't fitting for something as intimidating as Mega Garchomp.

My fix:

Everyone likes black and blue shinies! Plus, I really like the idea of a gray skinned Garchomp. I also darkened all the colors of regular colors for Mega Garchomp (darker gray and blues).