I have a suggestion for the color of the cover for the fifth book Terminal, and I think silver would be a good color for it . If you are already thinking of just black and white, I don't think silver is that much different, and I think it would represent sort of like the silver lining of all of the bad things that have happened in the series. That silver for the last book could symbolize the great outcome (I assume there is one) that comes along after this adventure that the Virals have been through. It shows that the last and final book gives you all of the good and positive things that came from this virus in the end.