Is there any other way to gauge coloration other than sps? The zoa addicts are about to jump all over me! [emoji12]
So here's my thoughts. I've never seen a nicely colored SPS tank with higher phosphates. I'm talking mostly acros and not so much birdsnests or digitatas. They tend to look good in some tanks irregardless of nutrient levels. Hence why most use them as a tester SPS coral to see if you can keep them. I've seen my fair share of tanks too... though I in no way can encompass the shared community experiences of viewing other tanks.
However, I've seen plenty of great looking SPS tanks at lower or higher alkalinity levels. To me, that removes the criteria that a greatly colored SPS tank is dependent on lower alk levels. Actually, the guy I learned the most from about SPS husbandry kept his alk at 11 dKh. His colors were amazing! Most though, keep their tanks around 8.5 dKh or lower because they are already running some type of carbon dosing and the general consensus is to keep your alk levels low while carbon dosing. So, there may be a correlation with lower alk levels with better colors in the general population but part of that may just be the coincidence that most of those lower alk tanks are also running some type of carbon dosing to control nutrient levels; therefore, better coloration associated with those tanks (zeo included).
I was hoping to see some evidence you've might have gathered to lend to the theory of lower alk levels correlating to better colors in SPS. I'm not suggesting you do this but it would be interesting to have you raise your alk to a higher level and leave it there for some time. I'm curious to see if your raspberry limeade, which tends to darken with higher alk, will eventually color back up if given time to acclimate to the higher alk levels. That perhaps maybe, it darkening is just a response to a negative stimulus, such as the change in alk levels, and then it will color back up nicely again after stability has been reached.
If you have any other observations regarding your coloration with your corals, I'd love to hear them. I find the topic fascinating and like to acquire as much information about it as I can!