Well, for the American editions, not too many colors are left. (I'm going by the colors of the :writing on the sides, because minus the last one, most of them have a ton of color)
First= Gold (Yellow) writing, multicolored cover
Second= Silver (gray) writing, multicolored cover
Thrid= Green Writing, multicolored cover
Fourth= Orange Writing, (mostly green cover)
Fifth= Blue writing and cover

The main colors left out here are red, purple, and black. I can defently see the last book being red (because someone is without a doubt going to die) so purple is likely. However, back in Ancient times, purple was the color that showed the highest, or the most important of something, and JKR is very big on ancient stuff (just look at the Latin in her spells), so this book might be red and the next could be purple! Yeah for overanalyzing!