When the release date (July the 16th 2005) was announced for
Harry Potter and the Half-Blood Prince, Scholastic (the American publishers) featured a generic purple cover in their
announcement.
Although this is not the official cover art, it is somewhat significant because the generic cover for OotP was blue and the cover art for that book also turned out to be blue, leading some to believe the American cover art for HBP may be purple. Purple is also a colour linked to royalty which would be appropriate given the title of the novel (Half-Blood Prince).
On the British publisher's website, Bloomsbury, the generic cover used is
red with the middle section hidden.
Thanks to
Mugglenet for the tip!