Maybe when Apple has completed the refunds for all the bumper purchases... That's the only reason I can think of on why they're not selling colored bumpers and only giving out free black ones.
I just assumed they stopped making the colored ones so that they could ramp up production on the black ones they are giving away and would eventually bring them back in september.
I placed my order two days before the big press conference, ordered a white one, and it came back saying it would ship by August 5, delivery by August 10. About a week after the press conference I received an e-mail from Apple stating I would not be charged for this white bumper. Last week I received an e-mail from Apple stating my bumper shipped. Checked the tracking at that time, and it said it would deliver on July 29.
Yesterday I decided to check the tracking to see where it was, and I was surprised to see it was already delivered to my house. Today, I looked at my bank statement and saw they refunded my card for the other two bumpers I purchased (black and green) - I had no intention of asking for a refund for these two as I felt the refund for the one white one was enough.
Anyway, I would think that once this program is over on September 30, Apple would probably start selling them again. Might even be sooner as soon as they catch up with all the freebies they are giving away.