I just started collecting mangas I wonder how am I going to prevent them from having yellow pages. Currently I store them in a glass shelf without a cover.
Although I have no manga, I do have a large comic book collection. I protect them with "comic book bags" (or sleeves) and anti-acidic cardboard. They are stored in special boxes designed for comic books. These supplies are also available and fitted for manga. You can store 1 or 2 books per bag. If two, you place them back to back, with an anti-acidic card in-between.
There are several vendors. The one I use is Bags Unlimited, which has been around since 1976. They have a huge number of "archival supplies" for comics, books, CDs, magazines, you name it.
I just read @Maroon's comment. Once yellowed and brittle, you can't do much about it. But the purpose of the anti-acidic paper is to minimize damage to the books in the first place by helping neutralize the acids in the paper stock of the books.
Yellow pages is a symptom of sun damage, more or less what happens when you leave something exposed to the sun for prolonged periods of time. A good example of this is if you ever go to a comic shop and they have posters in the window that are really faded, it's the minor amounts of radiation from the sun more or less bleaching those colors away.
A good way to combat and prevent this is to keep them in a bookshelf and keep them organized, and if there are windows in the room just don't leave the blinds open all day. It wont 100% prevent this issue, as @Maroon pointed out certain papers will just do this overtime. However speaking from my experience doing this is an easy and effective way to help prevent this.