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I have a X3 as well and was surprised how stupid the min/max labels are. The bottle is black (no problem with that) but you need to look down the coolant resovior to see the level. On the older BMW's, there was a float that would come up when you opened the cap. Being in hot coolant ment that the float would break and fall inside.

I keep BMW coolant at home because you can't purchase it locally. There is a company called OEM that makes coolant for these cars however, I rather buy genuine.

With that said, a top up with tap water would not hurt your engine. If there system has 6.5 litres and you add 1 litre of water, you would reduce the boil over and freezing protection but improve heat transfer. Unless you see -20 C or drive in heat heat, I doubt you would have any issues. In fact, I've run my cars with nothing but water and addivities.