Want critique?! draw more nude chicks!
You're doing beautiful photo studies, they are very helpful but some phenomenons are hard to notice and understand on them. In Indian Head you could add variety of local colours and colours of light as a result of bounced light. I would suggest you to look for veeeeery subtle colour shift when you will be doing next study. Master painters observed it very well and emphasized is very often. I am recently fascinated with Bouguereau regarding skin subtleness and all kind of shifts (hue, value, detail amount...).