I have Mystic Blue and it's definitely the hardest car that I've ever had to keep clean. I've had white, silver and a rust/orange before.
I've washed it every week; so far, and it shows the dust in 2-days.
I've resigned myself to alternating between washing it myself 1-week and taking it to the car wash the next.
__________________
5-2-08 e93-35 ED Montego w/Jade Grey, Step w/Paddles, PP, SP, CA, hot seats and iPod.Awaing re-delivery soon.
Debadged 330xi ED 11/14/05 Auto/Mystic Blue/Gray leather with Alum/PP/SP/CA/Fold-down rear seats/Alarm. Sold to Carmax on 6/7/08 with 50,624 miles on clock