Yes, applying henna is halal in Islam. It is encouraged for women to dye their hands and feet with henna, especially for occasions like weddings to adorn themselves for their husbands. However, it is not appropriate for men to use henna in this manner as it is considered a form of adornment specific to women, and imitating women is discouraged in Islam.

Sources:

1. IslamQA: Is there any connection between the presence of henna tattoos on the skin and jinn entering the body?
2. IslamQA: Ruling on men putting henna on their hands and feet

And Allah knows best.