Whew, I am glad to see all the interest in Ultrastable (where were you guys when we were making the stuff?) To begin:
1. Color Print Stability: Dyes and pigments have varying degrees of "stability" but in general, the dyes used to make color photographs (Dye-Transfer, chromogenic , Ilfochrome etc ) change (stain, lighten, darken, and/or etc) upon exposure to light. Some will even deteriorate (eg: Kodak's type C) when kept in the dark because the chemicals used to process the print remain in the photomaterials and continue to be reactive to their environment. A print made by the dye-transfer process by comparison, in which the processing chemicals are removed (Ilfochrome too), is essentially dark-stable. However, the final image is formed by dyes that will show changes in density, color and color balance when exposed to light. Not recognizing the important difference between dark keeping and light-stability was the basis for the myth that dye-transfer prints didn't fade.
Pigment assembly processes such as carbon and carbro use color pigments that are typically highly light stable. Ultrastable films used pigments that were developed for the automotive industry (no cadmium here) which required bright, non-fading paints for their cars. Which means that the ultimate measure of the display life of a color print made using highly-light stable pigments is not light-fastness (500+ years according to Wilhelm), but rather the physical integrity of the print. As has been pointed out, 500 years is a long time and it is difficult to fast forward time for testing purposes. Adhesion, cohesion, cracking - not to mention fungal and bacterial growth- are among the types of problems most likely to determine the actual (very) long term display life limits of these kinds of color print materials.
2. Ultrastable Pigment Films (CMYK)-- were made by coating a layer of pigmented gelatin containing a diazo-type sensitizer on a dimensionally stable base. It was a non-toxic, pre-sensitized, pin-registered version of the process Ducos duHauron used to make the first color print in 1869. Ultrastable pigment transfer prints were made on a variety of bases, including PET "Melinex", fine-art watercolor and hand-made papers. The color print films were designed to be used with high-resolution (300dpi and up) or random-dot separation negatives. The process is still in use today, with fine art photographers/printers such as Tod Gangler and John Bentley using the last of the materials along with freshly made (by themselves) films. The current Sarah Moon show at Fahey-Klein in Los Angeles, featuring color carbon's printed by Tod Gangler, is a fine example of the possibilities of the Ultrastable process.
In a few days, I'll post a PDF of the UltraStable lab manual for those who may find it to be of interest.