How to naturally fade very colorful paper?
I agree with Elmy's comment.  Not just toner, but most commercial inks are probably more stable than the paper they're printed on.  It would be hard to substantially bleach the printing without affecting the paper.  

If you just want the image as a light background, you could scan it, lighten it with image processing software, and print it as a faint background on fresh paper.  

Another approach you could try is give the pages a very light misting of white paint or semi-pigmented white wood stain.  It would be a challenge to get a uniform coating of the desired transparency; it would be worth practicing on some sacrificial pages.  It's also likely to affect writeability, so you might need to experiment with different kinds of pens, markers, and pencils to see what writes on it acceptably.

Your comment suggests something else you could try.  There is super-fine grit sandpaper (e.g., 2000 grit and higher), that's used for polishing.  You could try very gently buffing the paper with that.  It will remove some of the surface and the ink on it.  However, this will also affect the surface of the paper and change it's writing characteristics, so you might need to experiment with pens, markers, and pencils to see what works best.

I just tried that using 2000 grit sandpaper on a pretty page from a magazine (high quality photo printing on shiny coated paper).  The results were awful.  In this case, the colors might have been laid down sequentially, so rather than lightening everything evenly, it removed almost all of one color before attacking the next color.  Getting it light enough to be a background for writing, the result was ugly, unrecognizable blotches of color.  Also, the printing is only microns thick, so it's impossible to remove it evenly; by the time it was reasonably light, the printing was full of blotchy holes, where almost all of the printing had been removed.  So YMMV depending on the type of paper and the nature of the printing.