It is generally said that a women should not apply hina or hair colour during her menstrual cycle. The reason is that she is impure at that time and as such cannot get pure on getting bath once her cycle is over. These colors go with time and cannot be removed through taking bath. In other way you can say that if she has applied it before her menstrual period then it is ok as she was pure at that time. The same issue will apply when bath is due on her after copulation and she applies hina etc. before the bath at that stage. Kindly advise.
Since the matter relates to Ijtihaad there may be different opinions. We do not think that hina is something that creates a layer on the skin hindering its swashing. If somebody feels that way he may adopt this careful approach. However, we would not declare it necessary until and unless the given reason is established.