Lower light will bring them back to color. When zoanthids, corals, any xoozanthellae hosting animal gets too much lighting the xoozanthellae multiply dramatically. One result is that because algae are brown/golden in color the host animal will also appear brownish/golden. Also what can happen when exposed to too much light for extended time is what is known as oxygen poisoning, the host gets too much oxygen as a result of the over producing algae and expells the algae, turning white, though this is rare in zoanthids. Zoanthids are able to adapt and take on a darker coloration keeping the algae from getting too much light. This is what you are seeing, the zoanthid pigments/proteins turning dark, not necessarily darkening from the algae multiplication itself.
Its all relitive. As reefkeeperZ said, they are fine where they are, jsut a different color. Your others are doing better where theya re now so you have to decide which you want, a lighter yellow or a fast growing watermellon.
Any zoanthid can change color to a degree depending on placement to light. That's why we buy zoas that look one color in a pic but when we put them under our lighting they soon change.
Move them to lower light and they should color up again.
