Costume

In the first montage, the character gets dressed in business-wear, for a day at school. This represents a normal, teenage life- black is a mundane colour often worn for business purposes to appear professional.

 

When the protagonist takes the drug, and is at the party, she wears a simple grey jumper but is engulfed in red light. Her clothing had to be simple to emphasise the effect of the lighting. 

We distort the colour black, from something smart and professional, into evil and darkness. To highlight the aftermath of the protagonist's addiction to drugs, she wears black from then on. As you can see, the character starts by wearing grey, then black, as she spirals down into the drug world. Grey is worn at the party to show her tipping point- the first time she takes the drug. 

Black symbolises loss of hope and death which may foreshadow the end of the movie, as we can see the character's health and mental well-being deteriorating.