If you are just topping it off, I'd top off with the same color from the BMW shop. Some coolants are not compatible with others. However if you are going to drain and replace, I'd consider the Propylene Glycol coolant from Amsoil.
This coolant is similar to the Propylene Glyclol coolant Engine Ice, but costs half as much. Propylene Glycol has a much lower toxicity to pets and children than Ethylene Glycol and requires no special disposal procedures. It is compatible with all other major ethylene and propylene coolants if you have to top off with another coolant far from home. It is recommended for motorcycles by Amsoil and can be used up to 7 years or 250,000 miles. It is what I used for the last few years I owned my 137,000 mile 2000 K1200LT and what I will use for my first coolant change in my 2006 K1200LT. It has been sold in several Houston area motorcycle shops for the past 3 years.