- By far the largest factor is the choice of the colorant set for the inks, specifically, the chromophores of the colorants. In order of an organic colorant to absorb visible light, it must contain a highly conjugated system of double bonds. The way how these bonds are arranged together makes a big difference. If a single conjugated chain goes through the whole molecule , then breaking it in one place is sufficient to make the molecule colorless. On the other hand, in the system of conjugated rings, many bonds need to be broken before the molecule substantially changes its color. Thus, phthalocyanines are preferred as the cyan dyes/pigments and quinacridones for magentas; of course, carbon black which is essentially graphite with multiple benzene rings condensed together is very stable to light.
- Dyes vs pigments. Dyes and pigments often have the same chromophores, but dyes are water soluble, whereas pigments are water-dispersable. As a result, pigments forms a thicker layer on media and it takes a longer light exposure (or ozone, for gas fade) to completely break it down. However, there are also some very stable dyes that are now used in photo imaging.
- Organic versus inorganic pigments. Organic color pigments are based on aromatic structures connected by some kind of conjugation links. Some of these bonds are relatively weak and are easy to break by light quanta. Inorganic pigments, on the other hand, such as titanium dioxide (common white pigment) or ferric oxide (yellow-brown pigment) are very difficult to break by light. As a result, inorganic pigments are much more stable to light and are preferred for image permanence. Unfortunately, they tend to be duller and there is no good cyan, magenta, yellow, red and blue bright, chromatic inorganic pigments as they loose competition to organic pigments.
- Yellow versus magenta versus cyan. As yellow pigments absorb in the part of the spectrum that is the closes to ultraviolet, they are exposed to the largest energy of the quanta and photolyze the easiest.