Hi Maya!
I simply love the convenience the 
MagicBands bring to my life when at 
Disney World. Having that very cool band on my wrist is my keys to 
everything: from the resort rooms, to dining, to attractions, to shopping; it is the perfect "smart" band and a technological marvel Disney introduced to its guests.
Your question is an incredibly good one! Disney is always pretty good at meeting demand and having plenty 
MagicBand options to choose from in stock. I would say continue to check periodically at 
DisneyWorld.com from your MyDisneyExperience account, or check 
shopDisney.com to see if the colors you desire come available. Disney states that so long as you place your MagicBand order at least 11 days before your check-in date, that should give you enough time to order and then have it shipped to your home. Personally for me, I would extend that out and order your MagicBands at least 3 weeks before your trip, just to allow enough time for the shipping service to get the product to you in time before your trip!
Hope this helps. Have a Mickey Magical Day!
Terry