In printing, organic pigments are mainly used in the inks, so the shifting or bleaching of the color of a printing product due to the presence of UV light is usually just a matter of time. The use of organic pigments is justified primarily by their inexpensive cost compared to inorganic pigments. The particle size of the inorganic pigments is often larger than that of organic pigments, thus inorganic pigments are often not suitable to be used in offset printing.
In screen printing, the particle size of the pigment is not the limiting factor. Thus it is the preferred printing method for printing jobs requiring extreme lightfastness. The thickness of the ink layer affects the lightfastness by the amount of pigment laid on the substrate. The ink layer printed by screen printing is thicker than that printed by offset printing. In other words, it contains more pigment per area. This leads to better lightfastness even though the printing ink used in both methods would be based on the same pigment.
When mixing printing inks, the ink with the weaker lightfastness defines the lightfastness of the whole mixed color. The fading of one of the pigments leads to a tone shift towards the component with better lightfastness. If it is required that there will be something visible from the printing, even though its dominant pigment would fade, then a small amount of pigment with excellent lightfastness can be mixed with it.