The paper presents a method on how to predict the fading of particular printing colours based on action spectra. The action spectra are measured in a custom-built exposure chamber equipped with narrow band LED covering the visible light range. The action spectra are used to predict broadband exposure to ‘white light’ of a particular application. As a first example, the predicted fading curves are compared with actual fading data obtained using white-light LED. In the next step, the action spectra are applied to predict fading for the same sample set when exposed to a commercial, LC37-filtered Xe-arc light source, as specified in ISO 18937-2. The action spectra of colour print materials reveal that the main degradation reactions are caused by radiation below 420 nm, some degradation happens in the wavelength range 420 nm to 550 nm and very little degradation is caused by light above 550 nm. A severity factor to estimate the degrading potential of light sources for the material class of colour photographic and printing materials is developed.