At this year's New York Comic Con, Yen Press announced a second Highschool of the Dead Color Omnibus. I'm not sure if they gave a release date, but Amazon has it coming out on July 30 of 2013. This second omnibus will include Volumes 5-7 (I assume in full color) as well as the art book.
This is a surprising move, and I'm not too enthusiastic about it. As far as I know, there has been no announcement about Japanese color editions of these volumes. I suppose it's possible the announcement will be in the next issue of Dragon Age, but NYCC was almost two months ago. It doesn't seem like Fujimi Shobo would approve of Yen Press making the announcement that long before them. They could have said something at the various Japanese anime conventions, but then I think there would've been something on their site or in the magazine if they had. While I think more Japanese color volumes are very likely in the future, I can only guess that Yen Press is jumping the shark and publishing more color volumes without waiting for the Japanese versions.
What worries me is that it seems Yen Press will be coloring the manga themselves this time. In the first omnibus, they really only reprinted the Japanese versions. I never bought the English version and thus can't say much about the quality, but I've heard good things, including a positive review by Shouji Sato himself in a rare site update. I have very little faith, however, that they can produce the same quality if they don't have a Japanese version to work with. I sincerely hope they are at least in communication with Shouji Sato or getting some sort of instructions. If they work independently, I fear we will get the same poor quality they gave us in the regular volumes. The fans of the series don't deserve that again.
Perhaps there will be an announcement in the next Dragon Age, and this will just be a case of Yen Press being too eager to announce their English version. We'll just have to see in a couple of weeks.