I think the yellow might be from a couple of reasons the water was too high, therefore the Rockwool stayed too wet and suffocated it at the start, also could be because you haven't fed her anything until now and she is very hungry. I think you need a feed chart for those nutes you using, be easier to know how much of what you should be adding, a count of 550ppm could be made up of not the right amount of this and too much of that if you know what I mean... You gotta get the amount of each ingredient right, then check ppm if it too high then dilute and check pH and ppm again.