It should be completely obvious to a casual reader of major news sources whether they are reading news or opinion (or some other categories they might have). I suggest choosing a light blue/light green tint for different pages. With a physical paper, you have to physically move pages around, it's easier to know what you're looking at, you brought yourself there. But for online reading, it's frequently just a link, and so more obvious visual identifiers would be immensely helpful for the readership.