Potassium and lack of calcium appear to be the contributor to dark ash. Calcium and phosphorus lighten the ash....Coco is naturally high in potassium and low in calcium. My water is low ppm 10-40ppm or RO ,so almost no calcium.... It would appear over flushing coco with no calcium or phosphorus might actually NEGATIVELY impact burn quality. A friend of mine over feeds like a MF and flushes very little yet his bud burns pretty dam white considering. His water source is 300ppm plus- mostly calcium .....
I've read on weed forums phosphorus was the culprit for black ash but the science doesn't support that...... I too have noticed super purple bud doesn't burn as nicely. Purple leaves no issues but when the calyxes go super purp it doesn't burn great. Purpling can be brought out by phosphorus deficiencies-whether brought about by cold temperatures, genetics influencing its uptake or simple lack of available P.... Thanks for your insight
@MidwestToker you helped me figure out something that's had me stumped for a while now