Each of the main characters has a colour which can be seen in their clothes and possessions. Oliver is blue, Jordana is red, Jill is yellow, Lloyd is brown and Graham is black. As Oliver gets to know Jordana more and more red sneaks into his palette.
Towards the beginning of the film's prologue, Oliver is imagining what would happen if he were to die. In the daydream, Oliver refers to the man on the PA system as "Mr. Dunthorne". This is most likely a reference to Joe Dunthorne, the author of the novel which the movie is based upon.
As Oliver daydreams about people's reaction to his death, he mentions 'Mr Dunthorne' (and his quavering voice). This could be the teacher or perhaps the Headmaster. Mr (Joe) Dunthorne is also the name of the author of the book upon which the film is based.