Is it permissible to use black henna in general, and if one desires to get engaged, in order to cover white hair resulting from vitiligo?

It is permissible to use henna to treat vitiligo and gray hair. Indeed, dyeing gray hair is a Sunnah, as long as it is not with pure black dye. It is also permissible to cover vitiligo during an engagement, as long as it is not among the defects from which safety is stipulated.