A latent image ink particularly for use in printing forms such as games or colouring books. An offset lithographic press is used for imprinting a substantially invisible image on a sheet of standard paper. The latent ink used to form the latent image is a mixture of potassium ferrocyanide or other suitable colour forming iron complexing compounds, white ink and varnish. A developing solution such as ferric chloride or ammonium sulfate is subsequently added to the paper to render the image visible.
