Illuminating system for a photographic enlarger for making color reproductions, consisting of a projection lamp with a spherical or an aspherical reflection mirror and of an enclosure for mixing colored light. The enclosure is provided with an input and an output ground glass window, between which a system of movable color correction filter is arranged. Between the projection lamp and the input window there is placed a dispersive optical assembly consisting of a system of optical prisms, whose optical axes are parallel with the axis of the illuminating system and perpendicular to the direction of movement of the color correction filters. The prisms are paired equidistant from the axis of the optical system, each pair having the same refractive power. A single prism is also provided, lying on the axis of the optical system, having a zero refractive power.