I say, and with God's success: Henna is an adornment for women, and it is a Sunnah from the Prophet (peace be upon him), as Abu Dharr Al-Ghafari narrated from the Prophet (peace be upon him): "The best thing to change gray hair is henna and katam" in Sunan Al-Nasa'i, number 5093. It is originally a pure herb, and it is not known to be impure, so there is no objection to praying while applying henna, and God knows best.