Is henna considered apparent and forbidden adornment, and is it permissible for a woman to apply it during menstruation?

There is no harm in a woman dyeing her hands with henna; rather, it is مطلوب (desired/required) of her. The proof for this is the Prophet's (peace be upon him) saying: "Dye your hands with henna; one of you leaves off dyeing until her hand becomes like a man's hand." A woman must conceal her henna from non-mahram men, because it falls under the general meaning of Allah's saying: (And not expose their adornment). It can be concealed with gloves, and there is no harm in applying henna during menstruation or postpartum bleeding.