Duotone was a printing method quite popular in Doctor Who and Dalek annuals, as well as some Polystyle comics. It involved printing one halftone of an image over another, resulting in an image comprised of two colours, aside from white. Most images seen in licensed DWU material tended to be primarily red-and-black or blue-and-black.
A typical panel from a duotone comic story,The Orbitus
The method is rarely, if ever, employed in 21st century DWU publications.