Desirable by whom?
Since the car is leased I presume you're off the hook when it comes to resale, so what do you care if white cars don't sell well in a couple of years (which I doubt very much).
If you like white, go for it. Personally I find it a very impractical colour here in the UK where our roads are so dirty and nearly always wet. I try to go for colours I both like and that look good 6 days out of 7 instead of 1...the day it gets washed.
Bear in mind that an X6 is a lot of car to clean, so a colour that tolerates a little grunge ain't bad IMO. White is a great colour for showrooms and dry sunny climes, where it stays cool AND clean. For the UK, colours that change based on the angle the light hits the car are a lot more practical...greys, blues and silvers often have a 'hewn from solid' look that again IMO is very attractive and quite practical in terms of showing up the dirt.
I just had the rear arches of my grey X5 resprayed and fitted with clear protective decals under warranty because the stone chips were so bad. This 'grit blasting' is a feature of British roads never seen in other European countries. Coming back to the UK from Germany in winter, you immediately notice how much dirtier the roads are, so buying a highly sensitive colour that looks amazing when clean, but never is seems a little masochistic.