Sorry, missed your post.
Now, I'm no biologist or biochemist, but my understanding is that Coral exhibit colours as a means of absorbing wavelengths that benefit the zooxanthellae biologically so that they can provide nutrition to the coral, and then the coral provides them with a nice cosy home
If the level of nutrients in the water is enough to provide direct energy to the coral, the zoox brown up to prevent absorbtion of light as they are no longer required to provide nutrition to the coral.
If you think back 7-8 years when ULNS was all the rage, the idea was to starve the coral, forcing the zoox to provide most of the nutrition, so they had to exhibit bright colours in order to absorb as much energy as possible. Aminos were added to provide to the coral what the zoox did not.
So, if there are enough nutrients in the water, addition of more aminos will cause the coral to shut down a little and to brown up.
That's my understanding of the thoughts at the time, and most of the Amino product were developed during the ULNS 'phase'