A process for providing a security paper, in particular a banknote, with a coloured marking, comprising providing a photosensitive preparation on a portion of said document and submitting at least selected areas of said portion to a light beam, wherein said preparation is capable of forming a film on said portion and comprises a substance capable of producing colloidal metal particles under the effect of UV irradiation, and said areas are irradiated by means of an UV-light beam.