I'm curious if having undetectable phosphates and having 10-20 ppm of nitrates allow you to keep sps colored up, or do you need to have "some" phos.
IME yes. I have the Hanna Phosphate Photometer (big blue one) and I would routinely measure 0.0 po4. At the time my nitrates were 10-20ppm and my SPS colors were on point. I don't actually believe my po4 was actually 0, but low obviously. IME having po4 of .02-.04 along with 15ppm nitrate seems to be an excellent balance for color and growth under my lighting. This would probably vary depending on different lightings and photoperiods etc.