The present invention relates to the use of organomodified metallic nanoparticles preferably bearing at their surface a self-assembled monolayer of organosulphur compounds. A preferred use is in suspension form for dyeing and/or treating human keratin fibers. The invention also relates to a process for dyeing and/or treating human keratin fibers, by the application to the keratin fibers of such a suspension, and also to compositions used to perform such a process. The invention includes the colouring and/or treating of human keratin fibers, in particular the hair, with the invention suspension. Compositions of the nanoparticles with cosmetic active principles, and uses thereof, also are described.