From MangaHelpers:

This volume is a collection of short stories. The title story "Aka no Sekai" features a boys' summer camp out in the country. The main character, Regulus, is reluctantly attending the summer camp, where he shares a room with a cheerful boy named Miran. Soon after arrival, though, some of the other boys begin acting strange, while claiming they want to see the color red. Will Regulus and Miran be able to figure out what is happening before it is too late? [tethysdust]