Color is an "old media" vs "new media" issue. Color on the cover sells p-books, but is generally too expensive inside a p-book. Color cover images will help manage large e-book collections, but for e-books it is color inside the book that makes a difference. Since this is expensive for p-books it is currently only content designed to be on-line (and magazines) that use color extensively.
Good examples of this are the sci-fi periodicals. The ones I am most familiar with are
Grantville Gazette, where volumes 11-14 have extensive illustrations (and illustrations were removed from volume 1 because they were too hard to handle!) and
Jim Baen's Universe. These are both new publications, but I assume older si-fi periodicals are on the same track. They can be read on-line or off-line as an e-book.
I am still not sure whether I prefer reading these on the iLiad or the Nokia 770. On the iLiad I can see large images but in greyscale. On the 770 I can only see 800x480 pixels, but in stunningly good color. The strange part is that the illustrations are not central to the content (typically more relevant in GG than in JBU), but they do enhance the reading experience. I guess that I really want is double the 770's color screen (800x960) with at least a 12 hour battery life. The OLPC screen might already deliver close to this if someone repackaged it as an e-book device.