If it is common practice for women in my culture to put on henna for weddings or other occasions even where non mahram can see them, is it permissible for me to put it on if it’s considered normal?

No, culture does not override the laws of God. If the henna will be regarded as makeup and attract negative attention it can be considered haraam and should be covered in front of non-mahram men. If it is not regarded as seductive then it would be okay.