Is it old or is it dirty?
Propnomicon has some intersting thoughts on the process of aging paper. He notices a more brownish-green tinge when aging with walnut ink and a more yellow tone when aging with tea.
I don’t get the green tinge when dying with my inks – I switched to making my own ink a couple of years ago and it is a rich golden brown color. I did notice a green tinge on the walnut ink made by Making Memories, but I didn’t notice it on the ink made by 7 Gypsies. Perhaps it’s a brand related issue?
Propnomicon also mentions that he doesn’t like the greenish tinge because it implies to him that the piece is dirty or grimy. However, I prefer a moldy look to pages because the reality of aging is that cotton rag and vellum do not contain lignin and therefor don’t yellow. In fact, the page is going to stay relatively whitish unless it sees moisture damage.
When I need to make something look yellowed — like a pulp novel or an old newspaper I use liquid yellow watercolor. It works great as a dye bath (like the walnut ink) or can be brushed on to build up aging on the edges (as in a page from a book). I can also add other colors to it and do subtle washes to build up the tone (or even add a little mold).
In the end the technique you use depends on the feeling you’re trying to evoke. In reality, the basilisk page wouldn’t be browned and moldy through the entire page. It was torn from a sealed book. However a crumpled sheet of clean paper pried from Hermione’s hand wouldn’t evoke the same feeling as a scuffed and moldy piece of paper.
Propnomicon and I are lucky that the paper pieces we create get to be held and felt by the end consumer — as opposed to viewed on a screen. It’s a full sensory experience — the eyes, the hands, and even the nose come into play.