Discoloration and its causes
Hard rubber is a very stable material, but it is vulnerable to fading when exposed to bright light. The effects of exposure may not be immediately apparent, as the breakdown of the surface may continue for days afterwards and may not be visible until the surface is exposed to moisture. In most cases black hard rubber will first turn brown, then lose its gloss while lightening to a biscuit color. Note that while fresh hard rubber can be immersed in water with no ill effects, once light-damaged even cold water may cause it to suddenly fade or spot. For this reason, one should take appropriate precautions before soaking hard rubber pen parts. For more information, see Pen Repair Don'ts.
Because of the way hard rubber fades, abrasive removal of the faded surface can restore the original color. The problem is that such treatment will also remove chasing and imprints. Other, less destructive methods of faded hard rubber are in use, but as yet none is ideal and none can be used with colored hard rubber (more on reblackening here).
When celluloid discolors, it generally darkens rather than fades. This discoloration is not superficial, and is irreversible. While excessive UV exposure is not good for celluloid, discoloration is usually the result of reactive sulphur compounds released by deteriorating ink sacs. Long-term exposure to certain inks can also stain celluloid. The celluloid pens most vulnerable to discoloration are button fillers, since their barrels are tightly sealed and leave no room for the reactive compounds to escape. Note that the dark rings at the top of many Wahl caps are caused by the deterioration of the rubber disks placed under the inner caps (fortunately, only Wahl did this -- click here for further details). This is one more reason that one should be careful about even simple cleaning procedures: stable though the rubber disk may be when dry, it can produce highly reactive substances once wetted.
To prevent discoloration of celluloid pens, they should be kept from humidity, heat, and light, and they should be stored as to allow ventilation: no storage in airtight containers, and blind caps should be removed or loosened. If the pens are not to be put to immediate use, sac removal is a wise precaution -- though probably unnecessary if the old sac is thoroughly petrified. If they are to be used, sacs should be changed before they get too old, and silicone sacs should be used on particularly vulnerable pens, such as light-colored button-fillers. Antitarnish paper may help when used inside pen storage chests, since it protects both silver and celluloid by absorbing free sulphur. Placing small pieces of this paper within a pen barrel may prove an effective way to protect the celluloid from both the sac within and the barrel's own deterioration byproducts.
NOTE: while a well-known repair manual recommends use of old sacs as less prone to cause deterioration, this is in error. Rubber is not like lacquer or pressboard, which give off lots of gases and vapors when new and then stabilize. Rubber deterioration is progressive the old sacs are the ones that are the most dangerous.