Never, never, NEVER use pure black or pure white except on the pupils and the eye lashes. It looks really bad. I'm sorry, but it's the truth. Look at the official MLP characters. All of them are easy on the eye and use very soft, bright, pastel colors.
By the way, make the cutie mark as simple as possible. They're simplistic visual representations of the pony's special talent, not a fancy tattoo.