One of my fondest memories of being a child is the time I spent reading comic books, or comics, as we referred to them in the North of England at the time. British comics, in particular, were printed in high volumes with the cost of printing very much in mind. As a result, they were mostly either monochrome or duotone. After much searching, I found a good explanation, and a few good examples of duotone illustrations at http://todaysinspiration.blogspot.co.uk/2008/05/charlie-allen-on-other-black-and-white.html
There is now no sound financial reason to produce duotone illustrations, as far as I am aware, and my worry is that the format will die. You see, it doesn't matter how many forms of art I see, my favourite will always be duotone illustration. Out of the examples given, I particularly like the one with the amber highlight colour.