Metallic colors are very hard to repair, even for the pros themselves.
Depending on the age of paint, exposure to sun, temperatures, and other naturas conditions, the color and other qualities change.
Human eye is an amazing organ and does notice these small details. And it's even more picky with BMWs
If this happened to me - I would re-register my car color to BLACK (non-metallic) and paint the whole car.
Or any other color one might like.
Non-metallic paint is A LOT cheaper then metallic.
My advice is to consult a very competent painter and only agree to "fix" the scratch if he is absolutely sure that it would not be noticeable after he's finished.
If he has any doubts - "do" the whole thing. It's not as expensive as it might seem.
I've seen my cars go through the oven and MAN did I like it. It's like falling in love with your car, AGAIN.