It has been known for some time that the spectrum of radiation generated by partially coherent sources may be different from the source spectrum. In this letter we study the spectral changes produced on superposting partially coherent light emerging from two pinholes in Young's interference experiment. We find that whilst only small changes take place with narrow-band light, drastic modifications in the spectrum can take place when the bandwidth of the incident light is sufficiently broad. Our analysis distinguishes clearly the contributions due to diffraction and due to the state of coherence of the light incident on the pinholes.