The technique I have used is to paint the shadows first using just black ink, diluted to give grey shades. Darks can be gradually built up in layers. Watercolour paper is used as a surface.
Once a grey scale image is achieved it is coloured using coloured washes. I have used just three colours - process cyan, process magenta and process yellow. These colours are the ones used in printing and can be mixed to produce any tone required. The inks are mixed to the required shade then applied over the black and white image like a watercolour wash.