If you hang up a color print or poster, where it will be exposed to sunlight, you will notice that most of the reds wash out within a few months or years, while the blues remain stable. This is because the sunlight changes the dyes used to form the red images…why are these dyes so unstable?
I was looking at some family photos from the 1950’s-and just about all of the red tones are gone.
Are newer color prints more stable?
This isn’t exactly a WAG, because this is the way I heard in at some point in the past, but I don’t think it’s that the dyes themselves are less stable, but that the red wavelengths of light from the sun are more powerful due to less diffusion by the atmosphere. Thus they are more damaging to red tints, and so those fade faster.
I’m not sure if this is what Flying Dragon was trying to say, but: Pigments have a given color because they absorb certain colors from the spectrum present in white light and scatter the rest. In particular, red pigments absorb light in the higher-energy (blue) end of the visible spectrum and reflect the lower-energy part of the spectrum. So the red pigments, over time, will absorb more energy than the blue pigments, and that certainly can’t help their colourfastness.
That said, I don’t know why your family photographs (which were probably not stored in direct sunlight) had this problem.
It’s also true that most “true red” dyes, whether natural or artificial, will bleed more than other colors - red is never as colorfast. Light wavelengths dosen’t really account for that, does it?