I agree with these guys big time, great answers.
Zoas can, will and have come back to their original color or very near to it. I am in the process of nursing two very nice blastomusa colonies back to health. Back to zoas though, you have to place them low or no more than % 50 of the height of the tank. Why? Only from personal experience do I say that. This rule has worked with some crazy ricordea that I had, it's working with the blastos and it worked with some zoas a few years ago. The recovery could very well take 6 months to a year. Whatever causeed them to expel their zooxanthellae, ( the symbiotic single celled algae that lives within the coral that gives them their color/pigment ) must be corrected or they will continue to decline. Numerous factors can be responsible for this expulsion, but your zoas can be saved.
Direct lighting isn't necessarily what you're looking to provide to put them back on course to heal. Since light shock is but one of the many culprits, I tend to face the corals forward with indirect lighting with, of course, lots of current. Lack of current is also one of the many factors which cause bleaching. The biggest mistake one can make is to constantly move any zoanthid or palythoa colony from spot to spot. Doing so will only stunt growth and delay the recovery you are trying to achieve. Place it, leave it, inspect it from time to time and just leave it alone. Allow mother nature to do what it does naturally. Zoas can become seriously nutrient deficient while recovering, in short, it's like having a lowered immune system, making them even more vulnerable to other issues.
When you say your tank took a swing for the worse, what actually happened?
Best of luck to you..
Mucho