Yes, there are 3D-printed parts which discolor under influence of time or UV, with varying speeds and intensities.

PA12, PA-GF

 PA12 or PA-GF will start to show discoloration and turn slightly yellowish due to an ageing process. This process is an inherent property of the nylon material and is caused by a chemical reaction within the polymer chains. There is no treatment that can protect the natural color of nylon from discoloring over time completely. You can only delay the process by, for instance, applying a UV-protective finish. As an alternative, we recommend choosing a colored finish for your model like dyeing or spray painting.

TuskXC2700W, Poly1500, ProtoGen White, Xtreme

Whereas PA12 discolors over time, the epoxy resins used in Stereolithography discolor by direct UV-light more than any other materials. We recommend varnishing in order to slow down discoloration, or painting your model when wanting to keep a consistent color.

TuskXC2700T

A clear varnish is applied to your model in order to make it transparent. The varnish will protect your model from dust and slow down the discoloration process.

Example: The picture above shows a fresh ProtoGen White sample on the left versus a sample of the same material on the right which was exposed to direct sunlight for four days in a row.