You could use Gmic’s hue overlay masks:
https://discuss.pixls.us/t/hue-overlay-masks-my-first-filter/5000
Here I used the red mask (remember to set it to layer mode “normal” to see it):
Quickly applied the levels tool in Gimp (could have done better using curves):
This should work quite well for selecting the hair, if that is what you wanted.
Here’s the red channel of the image plus the red mask in overly mode, plus some curves adjustment:
Cheers