Washes for Underpainting for Notan

Since it's the shadow side of your subject that is to be blocked in, a fool-proof color for making the wash is Ultramarine Blue (or French Ultramarine Blue), though any color will work. Regardless of your medium when setting up a notan underpainting always aim for around value 5.   

Oils
 Mix a bit of Ultramarine with turpentine, Turpenoid or Gamsol.  Just a drop of linseed oil to the mixture slows down the drying time, extending your working time.  
     Another option for oils is an acrylic mixture of white and a darker color at value 5.

Watercolor
Develop a wash of Ultramarine (or French Ultramarine) Blue plus water in a small container.  Wet your paper, then allow it to become damp.  The wash for notan works better when painted damp in damp.  Let try completely before beginning the painting.

Pastels
You have all sorts of options when doing notan underpainting in pastels.  For the initial experiment,  it will be less confusing to begin with a light value, neutral support.  For the underpainting, three options are Turpenoid/Gamsol with oil paint, water with watercolor, or water with gouache.  

 The Kitty Wallis paper  accepts Turpenoid or Gamsol wash, but traditional pastel papers work better with watercolor or gouache.  Avoid cheap watercolor sets, though, because their gum content is so high, pastel won't slide over it.  Pelikan's Opaque Watercolor set is an excellent and inexpensive choice.  Or you might get a small tube of professional grade watercolor.

Gouache and Acrylics
For both media, a wash made with water  will work great, but a mixture of white and a darker color at value 5 is also an option.