Is henna considered an outward adornment, and is it forbidden?

There is no objection to a woman dyeing her hands with henna; rather, it is encouraged for her, based on the hadith: "Dye yourselves with henna, for one of you abandons henna until her hand becomes like a man's hand." A woman must conceal her henna-dyed hands from non-mahram men, because it is an adornment that falls under the general meaning of Allah Almighty's saying: (And not expose their adornment).