Pages turning colours?? Mmm, I imagine eventually all books in general yellow (or turn colours) with age. My Case Closed volumes look alright as do my Japanese volumes. For the most part my manga sits on a bookshelf unless of course I have it out to look at/read. Now, if you're in a different country than the US or Japan, it's possible that the publication of the manga in your country uses a different weight and brand of paper for printing. For example, I'm making a wild hunch that because the Malaysian manga publication is fairly cheap in comparison to the Case Closed or Japanese printings that they might use a cheaper, easier to get paper, which could account for pages yellowing (or browning) with age fairly quickly. So there might honestly be nothing you can do about it, other than keeping your manga from being exposed to sunlight by keeping it safe on a bookshelf. (Sunlight exposure changes the colours of many things, not just book pages.)
Another thing could be the material of the paper reacting to the oils from your hands. You can try washing your hands with soap and water before reading your manga to help protect it. (I don't do this when I read manga personally, but occasionally before picking up my game controllers to play video games, mainly to get a better grip on the controller.)
And of course, if you ever run into the problem of your pages bending, simply don't open the book quite as much. You don't need to open up the pages completely in order to read its contents. (As a matter of fact, way back when in the early 90s in the US the Mixx publications would end up losing pages from the binding if you did this. So I suppose my habit of not opening pages all the way comes from that. Many of my old print Sailor Moon mangas have loose pages due to crappy binding.)
On a side note, my movie collection is complete! (For now anyway.)