First of all, this isn't me doing paintless dent repair, way outside my skill level. I can only echo what they were discussing and what I have seen on my vehicle when I had a few dents repaired that way.
The process of the dent creation and then the dent removal causes the paint to ripple. The guy that did my vehicle used a polishing compound to slowly blend in the ripples but they don't go completely away albeit almost impossible to see. Different paints and different colors have direct effects on how well the ripples blend, according to the discussion I was reading.