1 April 1871 the first Danish Postal Cards are introduced, and the imprinted stamps are the same type as the bi-coloured stamps. But the Postal Cards are only printed in one colour, and are therefore not bi-coloured, but anyway the Postal Cards are also called "bi-coloured".

4 skilling postal card with "bi-coloured" stamp imprint

In 1870 the first Danish stamps in two colours are introduced. This type is still today known as "the bi-coloured" stamps.

4 skilling bi-coloured stamp

If you want to know more about the bi-coloured stamps, please look at the websites of Henrik Mouritsen, Leif Oerndorf and KPK (Københavns Philatelist Klub). And there is lots of literature on the subject.