Just out of curiosity, and I'm not trying to be rude. Based on what evidence, in your experience?
In the past, I've had a similar experience, but I've always had the rainbow colors come back. Upon obtaining them, I place them under a modified 20k spectrum, roughly peaking at 100 par. For those that have turned orange, I've noticed newly formed polyps display the original rainbow patterns,etc.
I've noticed feeding the ones that have retained the rainbow and intensity, including the individuals that have shifted to mostly orange, greatly benefits these corals. It helps retain color, and help the ones that have primarily lost it, regain it. As
@Randy Holmes-Farley has stated previously, corals obtain trace elements from feeding as well as dosing trace elements.
Although it's a multifaceted approach, they can regain their colors. Par, spectrum, the amount of UV radiation they receive, quality food, adequate levels of nitrate but mostly phosphate, and the maintenance of major and minor Trace elements, all play a role.
Out of curiosity, for the people who have lost coloration in their rainbow "micromussa", what are they doing to regain, and/or maintain? I'm genuinely curious. I love this Forum because we can all come together and share our experiences, in order to maintain these animals that we all cherish. I genuinely am curious what people are doing so we can all help each other.