Go Tanzanite, it's a awesome shade of blue.
https://g05.bimmerpost.com/forums/sh....php?t=1619956
The purple effect you're referring to comes out with the Arctic Grey color. Most of the time Arctic Grey looks like a light blue/grey, and the purple affect only appears from the hood in direct strong sunlight.
I've got Arctic grey, and love it - especially with the shadowline trim and black front grill.