Inks which change colour, discolour, tint or disappear after exposure to water or moisture. Reversible hydrochromic inks become transparent after wetting, and turn white again when dry.
Thanks to this property, it is possible to hide information or an image which appear after contact with water. In their irreversible version, the inks can be effectively and permanently washed off from the print surface.
Tinting hydrochromic inks are used e.g. for printing colouring books for children. They are also used for protecting securities against forgery.