Thank you all for such nice comments - I love toned paper as well. In fact.... a long post on toning your own paper is in the works.
There are almost as many ways to tone your own paper as their are artistic minds - shellac, as Rob Liberace speaks of, hide glue tinted with water-based media, raw pigment, even smudged charcoal pushed into the paper with paper towel.
Right now we've got an 8-foot by 15-foot mural "old master copy" in the works of a section of Michelangelo's Sistine Chapel ceiling, and we're experimenting with all kinds of toning methods for the paper for the cartoon study. It's tremendously interesting, and the challenge for all of us is to keep this fascination a utilitarian passion - always to enable the work, never to steal time from the work
The sheet is being used as reference up at our natural light studio - I have the model back for an extended portrait session, and I spread out all of my old drawings of her to help me pick the pose/composition of what will eventually turn into a portrait grisaille.
If I don't remember to post that particular sheet, I'm working on a post that shows piles of small model studies, and a description of how I use them, how they helped all of us grow visually....