I had a new Xenon low beam bulb installed on my 2000 750iL by a local BMW dealer. Now my bulbs do not match in color. Both original bulbs had a blue-ish hue, but the new one has more of a yellow-ish hue. The service manager insisted that that all new bulbs have this color, but will change to the more blue-ish hue after about 300 hours of use. I find this hard to believe. He also told me that there is only one model of Xenon bulb available. The part# was 63-21-7-160-806. Anyone have any thoughts or experience on this?

Yes, older xenon bulbs do change color over time. If it really botheres you that much you could always buy another 140$ bulb for the other side.