Not an expert, but potassium is to bring out purple and blue.
Are you noticing light colours from montipora caps? If so, then it could be a sign that K+ is needed.
According to your tank profile, my guess of colour losing may be due to your vinegar dosing which brought down no3 and po4 so quickly.
led may contribute to that also.
Give a bit iodine and feed a bit more to see first.