The beautiful coloring of shells won’t last forever and especially if you put them outdoors in the sun. I have learned this the hard way!
I bought a couple of large pink conchs about 20 years ago (I no longer buy seashells) and I’ve been putting them on my sunny porch railing in summer. Lately I’ve noticed that they are looking more faded and not quite so pink and it’s because of the sun! Mollusks spend their time living down in the water, out of the sun so it stands to reason that shells are not made to withstand sunshine.
On the other hand, if your starfish or sand dollars need brightening, just set them out in the sun for a while. They are much more brittle than a sea shell and be sure to bring them in when it rains because water will make them soft.