This is the understanding I've picked up over the years from hobby experts and scientists. Adam Blundell has done a lot of study into this. Hopefully he will chime in.
1. Too much light encourages more dense brown Zoothanthellae growth as a method of UV protection.
2. White is the color of a coral when it has very little Zooxanthellae, either because some stress, like transport or bad chemistry, caused it to expel most or all it's Zooxanthellae (sometimes called bleaching) or because it was in an environment of such low light that Zooxanthellae had a hard time growing.
3. Many of the coral colors are due to a Zooxanthellae combination of different color varieties. You know, how yellow and blue make green.
4. Innumerable variables combine to form environments that are quite different from one tank to another and even from one ocean to another. It's my understanding that Zooxanthellae, belonging to the same algal group as Dinoflagellates, can; 1)change growth, 2)vacate the host and/or 3)repopulate a host coral in response to the environment. I've seen coral that slowly picked up the color (Zooxanthellae) of another coral in my system. As was mentioned already, yellow Zooxanthellae is uncommon. When a yellow coral (or any coral) moves to a new environment, to a new tank, it can change more than the color of it's host coral. I've seen changes in growth patterns and shapes too.
Your original question was, "What can I do to get my bright Whites and bright Yellows back in my corals?"
My best answer, "Stress them and turn down the lights. 
"
Your second question, reducing the blue, certainly can make a difference. Have you seen the pics I occasionally post showing color and intensity at 4 different ocean depths? They illustrate the wavelengths of light that coral live under and also point out how color cannot help but change when the coral is bombarded with a new set of wavelengths.
Aloha,
Mark 
Edited by Mark Peterson - April 07 2016 at 7:49pm
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