Range of inks, visible in black, used for printing various shades of grey colour, which, when being wet with a small quantity of water or simply with a wet finger, will become coloured (several colours are available).
A printed document will easily switch from black to a colour just by wetting this document. Such a document will be authentificated and distinguished from a counterfeited which will not show this reaction.
Those inks are only printable by letterpress or dry offset. The coloured reaction is made once and is irreversible.