Do you mean shade/highlight the hair or are you talking about the blurry grey area around hair and head?
If the hair shape is on a separate layer, you can control click the thumbnail and invert the selection (ctl/cmd + shift + i). Then you can paint even across the selection line, but you won't be laying down any color outside your selection. You can only paint within the selected area, which is everything but the hair. Or if it is within the hair shape that you want to paint, just select it; same principle applies.
Of course, if it isn't on a separate layer, you maybe indeed need to create the selection and to maintain the crisp edge, yeah pen tool is best. With a shape so simple and contrasty, you could use other selection techniques as long as you zoom really close and check your edges.
IDK if this helped. If not, try explaining to thick-headed me again. Or maybe someone else will recognize what you need. :mrgreen: