In July 2015 I decided to get rid of some old origami models which had faded beyond looking nice. On these Sonobe units retrieved from some modular assembly, one can well see the deterioration of colors in inexpensive copy paper over time. The parts with vivid colors were hidden below additional layers of paper and so were protected from most sunlight. The bleak parts are those which were exposed to light (it’s probably ultraviolet radiation which is responsible for most of the decay).
Since I sign and date most of my works, I was able to check when the model was folded — November 23rd, 2002 — which means this paper was aged over 12 years. This is paper from a 9×9 cm note cube, most probably from Herlitz.
In early 2021, I checked on some of my older modular origami models (in particular, a Menger Sponge folded no later than July 2004, meaning it was over 15 years old), and the once vivid colors were all reduced to a homogenous, bleak yellow-grey hue.
Likewise, in the pictures above you can see the effects of paper fading over at least 14 years (Decorated Icosidodecahedron folded no later than 2006, first picture from 2006, second from 2020). Note how the orange parts have become almost indistinguishable from yellow. Paper type and quality is similar as in the models mentioned above.
While I don’t yet have any models that old folded from high quality paper, even the oldest of my tessellations, which are by now over 5 years old, folded from Tant, Elephant Hide or Khepera show no sign of aging so far.