milk/heat/saliva rash

I really feel you on this, it can be quite worrying to see the skin looking uneven after a rash. What you are describing, the two-tone skin after inflammation, sometimes happens as the skin heals, but since it has been going on for a few weeks now, it would be good to bring her to see the PD just to be sure and to get the right cream or advice for her neck folds. In the meantime, keeping the area as dry and clean as possible after feeds can help prevent it from getting worse.