Light is a primary culprit. Light, especially UV light breaks the chemical bonds of colors and breaks them down. Blues are very susceptible to fading. Red violet ( a reddish hue) turns quickly to brown. Other hues that include blue change to different shades. Reds also fade fast, going to oranges and browns. Green becomes yellow as the blue burns away, then ultimately the yellow fades to white.  

For a simple test just place flower petals in bright sunlight and watch which ones have changed color the most by the end of a day. 

In combination with light, heat also plays a part. It may just speed up the degradation of the bonds that make colors.

Some shells, such as cowries, may also lose their original color as the shells are scrubbed by sand, and salt water works on scratched areas. I was amazed at how swiftly a once beautiful, shiny Annette's Cowry first became cloudy, then the brown degraded first to blue then purple, then pink. The process took less than a week.

I had some very fine Fusinus dupettithouarsi that still had the heavy periostracum on them. I managed to bleach the periostracum off and was delighted to discover that there were beautiful red violet lines at the growth stops. But when I left the shells to dry after rinsing off the bleach water the red violet turned brown within an hour in the sun (about 80-85 degrees f). After a full afternoon the lines were nearly invisible against the stark white of the rest of the shell. I was much more careful to clean the rest in shade and the color is still almost as nice 10 years later as it was when I cleaned the shells. 

Dale Snyder

On May 9, 2020 at 11:49 AM Louis Wang <[log in to unmask]> wrote:

Dear all,

Is there any articles or your own explanations about why some beached shells(Cypraea, Marginella, Oliva) can keep their gloss but the colors are always yellow and orange, sometimes even pink no matter what kind of colors they used to have?

Thanks and kind regards,

Louis

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