Sketching on toned paper is something I discovered just last year. It's a great way to practice your shading and here's why.
The paper already has a shade to it. This means that whatever highlights (white ink) your adding to the sketch will pop significantly more than if you were to sketch on a blank white paper. You also don't really need as much marker work to define the shape of the object.
In this video, I sketch a Lambo style, mid engine car on toned paper. Don't ask me what type of paper this is because I honestly don't know. I just happened to find a stack of it in the closet. My wife used it for something as some point.
Anyway, if you have any questions, let me know!
//Marouane