Likely the lack of nutrients if you're running Zeo and have a PO4 of 0.009 ppm(what test/titration/meter are you using?). Spectrum could be it too. Are you utilizing the red leds on your Radions? Maybe add a blue/magenta stunner strip to further enhance the reds and blues.
I'm not extremely familiar with Zeo, but at a certain point, things are supposed to pale out, and then you start dosing various other things like AA, Pohl's, etc to deepen and enhance the colors. What Zeo products(or equivalents) are you dosing at this point?