Mixing coolant types is a bad idea in general. Some mixes gel up and cause real issues. That pink stuff might be G12 which is what my VW uses. I don't think BMW E39 coolant is G12, but I'm not too sure on that. It's not the color that's the problem its the type that matters. Although the different colors might be what alerts us to the differences. Call the dealer to be sure or read the labels carefully.
Thanks for the replies. I decided to go ahead and use it, as I had already bought the coolant, and I wanted to finish the job today.
I did a complete coolant drain although I believe 2-3 litres of the old coolant are still in the system. If I am told definitively that using the MINI coolant is a bad idea, I'll drain it again and replace with the blue stuff.
Ok, I've learned that the pink stuff is definitely not ok (it is for MINI diesels). Fortunately I haven't driven the car (just started it up to test for leaks), so I've do another full drain and replace with the BMW blue stuff.