The Boston Globe reports that Scholastic is pondering the size of the first run of Harry Potter and the Half-Blood Prince. I had not know this about the Order of the Phoenix:
Scholastic spokeswoman Kyle Good said the publisher has not yet determined the size of the first printing, but is mindful of what happened with "Harry Potter and the Order of the Phoenix." The first printing was 6.8 million, but the publisher had to rush back to press for another 1.7 million copies before the release date, and added 800,000 more almost immediately, for a total of 9.3 million within the first few days of publication. Five million books were sold in the first 24 hours.A post at Godric's Hollow speculates about the cover design:
Anonymous writes: "When the Scholastic (US publisher) website was updated for the Half-Blood Prince release date announcement, a generic, purple cover was used to signify the book. It is not the official cover, of course, but it is significant because they used the exact same design to stand in for the not-yet-completed Order of the Phoenix cover, but that one was blue, just like the real cover turned out to be.
Purple is the color of royalty, so it would be quite appropriate considering the title of the book. Purple also happens to be a color that JKR seems to favor.