HMG, welcome to
RP!
Epending on the image, it could be best to create a mask of the hair so that you could isolate it on a separate layer where you could:
- Desaturate it to turn it gray
- Invert the color
- Use Replace Color command
- Use Selective Color Adjustment
- Make a Hue / Saturation Adjust
- Levels Adjust
- Curve Adjust
......probably anoth half dozen options.
Perhaps if you post the image here you would get more specific advise and detail.
Regards, Murray