The colors just indicate whether the guardian is Normal, Mighty, Superior, Accomplished or Legendary. I'm not aware of any differences between, say, having a Mighty and Legendary guardian, besides the appearance. Legendary guardians are by far the rarest.
Spinel is a special MyShop guardian and she can be any type: Power, Magic, Sense or Charm. What type of Spinel you get will show in her name (e.g. Magic Spinel) and the glowing symbol beneath her sprite.