Al-hādiyāt is the plural of hādiyah: here the animals at the front of the quarry, not human guides.
ʿUṣārat ḥinnāʾ signifies the extracted juice or colored substance of henna. Shayb means grey or white hair, while murajjal describes hair carefully combed and arranged.
The comparison is not about redness alone. It is specifically about crimson marks standing out against an orderly, pale background.