The present invention relates to a device and method for selectively bleaching or dyeing a plurality of locks of hair. The device basically comprises a selector comb for selecting the locks to be treated from a portion of hair, and a container provided with at least one compartment that can internally house at least one liquid or pasty product for dyeing or bleaching. The selector comb is partially introduced inside the container and is movable inside and along same, being coupled thereto such that in an operating position of the device, the movement of the comb takes place, causing the gradual introduction of an increasingly longer portion of the locks selected by the comb inside the container at the same time as the gradual opening and communication of the inside of the compartment with the portion of the locks introduced therein, said portion being exposed to the liquid or pasty product.