All images posted here
Sally Thomas: Read Orrin Pilkey's How to Read a NC Beach, Chapter 6 pg 113 on the subject of brown and black shells. These are secondary colorations and a result of the reaction of iron to either oxygen (brown or rust of iron oxide) or sulfur (iron sulfide). This may be your answer. Good luck, Sally
Sharon: My dark olives appear to be fading to a lighter color, as they've sat in their box.