From what I heard the dye would be a chatzitzah when she goes to the mikvah
I don’t really know the halachos but my friend who is married wanted to dye her hair ombre brown to blonde and she wasn’t allowed cause it would be a chatzitzah. But her mom got a heter to dye her grey hair brown because it’s her natural color. So I think you have to ask your local rabbi cause two different rabbis gave different psaks