Neil, This is a question that collectors have debated back and forth for a long time and I'm not sure anyone has come up with an answer that satisfies all.
What you're refering to is called "patina"--the markers and hands turning color to a pale ivory in some cases to a coffee color to a darkened brown in some cases.
Let's look at the common explanations and ..."explore".
There are those that feel it's exposure over the years to UV radiation (sun light).
However, this does not explain why some watches that have had heavy exposure over the years maintain white markers while others (as has been reported) that have been kept in darkened conditions--collections in bank vaults seen to feature some darkening of the markers.
Another explanation has been lack of service--allowing moisture in causing the indices to "turn" (as well as discoloration of the dial in some cases.
I think in some cases there's a bit of merit to this one.
Some feel it's the natural drying of the bonding agent that holds the tritium and phosphors together in the markers. The bonding agent accounts for about 90% of what we see.
I'm getting more and more comfortable with this one as it's known that over the years Rolex has played with the mixture used on their dials.
I do think preservation is related to maintance as I've seen some pretty ragged examples that were simply abused over the years, but mixture has got to be a factor also.
Having said that I reserve the right, like my wife, to change my opinion with little or no notice.
