There is described a printed security element comprising a rainbow feature exhibiting, at least in part, a gradual transition from a first colour to a second colour distinct from the first colour, the rainbow feature extending over a colour-gradient area of the printed security element where first and second printed patterns are partly superimposed or juxtaposed, each of the first and second printed patterns comprising a first, respectively second set of linear or curvilinear elements. The first and second printed patterns are printed in register one with the other by means of two distinct printing plates so that the first and second sets of linear or curvilinear elements are partly superimposed or juxtaposed in the colour-gradient area and thereby generate the rainbow feature, the first printed pattern exhibiting the first colour and being printed by means of a first printing plate and the second printed pattern exhibiting the second colour and being printed by means of a second printing plate. Furthermore, at least the first or second printed pattern exhibits, in the colour-gradient area, a modulation of line width or line structure such as to cause, when superimposed or juxtaposed with the other printed pattern, a gradual transition from the first colour to the second colour. Also described is a method of producing the aforementioned printed security element.