The 500 duke was introduced in 1979 with the script tang stamp. That tang stamp was changed to block letters some time in the 84-85 years. sorry, best I can do for you is your knife was made sometime in the 79-85 years. Note: this timeline is from the Blades guide book.
There are a few guys here who are pearl collectors. That is one of the few I have. Maybe one of them can answer your coloration question.
JB