This sounds like The Fog by Caroline B Cooney, published in 1989. It was part of the Point Horror series when I read it. I couldn't tell you what the cover looked like but it's set in Maine and the main character, Christina, did have the silver, gold and brown hair.
It's since been republished as Fog. Plot summary from Amazon:
Christina Romney is thirteen, with a personality that matches her
unruly but charming tri-colored hair. She is about to start seventh
grade, and for kids from Maine’s Burning Fog Island, that means
leaving their little white schoolhouse for regular classrooms and life
on the mainland. Everyone assures Christina it will be a fantastic
year. Mainland school offers great advantages, after all:
extracurricular activities other than boating and fishing, a gym, a
cafeteria, and more kids her age. Best of all, this year the boarding
students will live at the historic Schooner Inne, a former sea
captain’s house (and now a bed and breakfast) recently bought by the
school’s charismatic new principal and his wife, Mr. and Mrs.
But Christina is apprehensive. She adores the wildness and excitement
of her island life. Boarding with her island friends will surely help:
Anya, a beautiful senior, fifteen-year-old Benji, the aspiring
lobsterman, and his crush-worthy younger brother Michael. But
Christina’s apprehension sharpens when Benji and Michael aren’t as
friendly as they used to be on the island, and Anya starts acting so
strangely it seems she is slowly losing her mind. Christina is
increasingly certain the Shevvingtons are behind all of these changes.
But no one else can see the Shevvingtons’ eerie behavior—not other
teachers, not her parents, not even her fellow island kids. Is Anya
the one going crazy in the Schooner Inne—or is it Christina?