The typical wardrobe for slasher horror films tend to have the main cast distinguishable with the time period of the film. With the attempt to make the audience relate to the characters they expect to have mainstream fashion and media.
The most important asset with costume is that the main villain/ killer defines who they are, and usually relates to the way they act.
The killers quite often tend to dress really raggedy as if they don’t care for their appearance, and they are never seen wearing anything fashionable.

So, the villain in the movie is Flint. I am going to have him wear black to connote that he is evil and the bad guy, even though the audience doesn’t know. But to sort of challenge the conventional raggedy look, he is going to look good and clean, like a normal teen that likes to hang out with his friends.
Abby is sort of the “dumb blonde” character, so she is going to be wearing something cute and girly, something with pink to connote to her femininity.
Jordan is just a normal chill guy, he is going to wear what most kids wear at the moment, just a T-shirt with joggers, plain and simple.
Riley is in a relationship with Taylor, so to symbolize her innocence and infatuation she is going to be wearing white, with something red as well, to symbolize romance.
Taylor is the one that is going to get killed, so he is going to be wearing red to highlight the fact that he is in danger, also the color red connotes to blood.