First off, I don't dose potassium myself, although I have a bit of interest in doing so. As far as I know, there isn't a cheap, reliable way to test potassium levels This would be a dream machine,
Palintest Photometer 7100, but I'm not sure it would even work with a bottom range of .5 mg/l. I believe those that do dose it responsibly, starting their dosing much like carbon dosing. Start with small amounts and slowly increasing over time until benefits are seen, and reducing if negative side effects are noted. Like I said, I don't dose, but from what I've read, potassium increases blue coloration, not red.
Increasing red coloration is often done by limiting nutrients and finding that right light level, usually high light. If your nutrient levels are already basically zero and they are exposed to "the right light level", you may need to increase nutrients by feeding more often or utilizing a carbon source like sugar, which will darken paled corals in ULNS.
