I remember reading a story when I was young (so 1990's) about a group of kids, possibly middle school or high-school age, that end up traveling to an alternate reality that's in black in white. I believe they end up going to it while they were in school, so they appear in another school on the other side. The kids stand out because they were in color while everything else was in black and white.
Eventually the kids start turning black and white themselves. It also turns out that if they completely turn, they're stuck in this alternate world forever. They end up escaping by finding an item buried in someone's bag (or purse, unsure) that hadn't changed yet and use it to get home.
I remember it feeling sort of Goosebumps-esque, but I looked through the list of books and didn't see it.
I read it in the 90's, possibly very early 2000's. It was an American story and in English. I don't think it was a full-length novel, but might have been shorter like a Goosebump book, but I'm fairly certain it was not in a collection, it was an individual book.