I'd go for the original (if that's what it really is) black too. Only because I like it though.
However there is an alternative viewpoint. I'm currently doing up a 300 year old building which has been in a pretty much constant state of change since it was built. There are many occasions when you look at a room and ask yourself 'at which point in its life do I want to work to'? Original would be, well, original but not necessarily best - for instance it would have no gas, electricity or running water. Oh and it would be a bank - which might not be so bad!
I think there is an equally valid point in favour of doing a bike up as a tidy version of what it was when you got it. Changes, mods, etc are all part of its history and their history is part of the fascination.
A friend of mine has a small but exquisite collection of Japanese swords. The pride of his collection is a 700 year old spear sword. It is in a wooden box inside the lid of which are columns of Japanese writing describing the entire history of the blade - who made it, when, for whom, for which battle and the names of everyone who has ever owned it. It was one of 250 and there are three known examples left. A sword museum in Japan has one, the Smithsonian Institute another and then there's his. He freely admits that he shouldn't really have it, that he's punching well above his weight to have managed to acquire such a significant piece, but he has. One day I asked him if he was going to have his name added to the list in the lid and he looked at me as if I were mad - to desecrate this piece of history. I pointed out that, in my opinion, to omit his name from the list would be destroy the integrity of the history of the piece. Who was he to suddenly decide that collectors of the future should not know this period of its history?
Not everything has to remain original it is interesting to see an original object but only interesting. In practical terms it is somewhat more interesting to see how people have changed things from there original state because that better reflects the tastes and necessities of their times.
I'll give a final example, one that many will know about (but it's worth a Google search and a visit to Brooklands museum if you don't). The Napier Railton, built in about 1933 with some old Railton limo chassis and a WW1 Napier Lion aero engine, it is the Brooklands lap record holder, set dozens of speed records, was even used as a test bed for aircraft braking systems whilst owned by GQ Parachutes. It's still run and it's my favourite motor vehicle on Earth by a fair margin. Part of the fascination for me is its rich and varied history. I'm sure there are those who would insist that the engine would have been better off left in the aircraft and the chassis in the limo. Maybe GQ Parachutes shouldn't have put a pair of huge disc brakes on the back either - but I'm glad they did.
Chris, it's yours. Paint it whatever colour you want. BMW sold it, if they'd wanted it to stay the same forever they would have kept it! Main thing is use it and enjoy it.
nobody needs another museum piece 
bit like saying your going to "restore" the mona lisa 
- luckily he decided to save it 
