OK colors for the hair underpainting... are not the same as the flesh colors (unless they have rosy colored hair - oh my). What I do is choose colors I see in the reference and try and use one or two or three colors from the flesh toned underpainting palette. So maybe with a sandyblond hair - I might use burnt umber, a sandy brown color, cinnamon, and a yellow ochre. The more colors I can use from that flesh palette the better - for color harmony.
Here's some samples below. I tried to figure out what colors I used... because I didn't have them written down.
This is an example of dark hair.. that when lit by a warm light has a lot of red tones. So I was able to use burnt umber and Pompeiian Red in the colors. To which I added a greenish brown and some soft blue highlights.
This is ummmm my hair which is that ash color. I managed to get burnt umber in there... and then made sure I used some cinnamon in some highlighted areas in the later stages.
This hair is a strawberry color so it was easy to get my underpainting colors in! I left out the Pompeiian Red in Place of an orangey-er color.
This last one has some burnt umber, cinnamon and ivory in it... but again the Pompeiian Red was replaced with a golden brown color. I think I galzed some greenish color in areas as well.