It is not permissible for a woman to show her henna-decorated hands to non-mahram men
Question

Is it prohibited for a woman to put henna on her hands in front of men?
Praise be to Allah.A woman’s putting henna on her hands and feet comes under the heading of adornment that it is not permissible to show in front of non-mahram men, because Allah, may He be exalted, says (interpretation of the meaning):
{and not expose their adornment except to their husbands, their fathers, their husbands’ fathers, their sons, their husbands’ sons, their brothers, their brothers’ sons, their sisters’ sons, their women, that which their right hands possess, or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women}
[an-Nur 24:31].
Shaykh Ibn Baz (may Allah have mercy on him) said:
If a woman applies henna to her hands or feet, she should cover them in front of people; she can cover them with socks or clothes, because that is a cause of fitnah (temptation).
End quote from Fatawa Nur `ala ad-Darb, 17/272.
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said:
We should understand that henna comes under the heading of adornment that it is not permissible for a woman to show to anyone except those to whom Allah has permitted her to show her adornment. In other words, she should not show it to non-mahram men. If she wants to go out to the market, for example, for some errand, then she must wear socks on her feet if she has applied henna to her feet, and by the same token she must cover her hands, although covering the hands is what is prescribed for a woman if there are non-mahram men around her, whether she has applies henna to them or not.
End quote from Fatawa Nur `ala ad-Darb, 7/2.
And Allah knows best.