My wife is on birth control (approved by a Rabbi) and when she takes a break after 6 weeks her period isn’t a normal flow. Rather she only stains and it never become a classic period that she would get if not on birth control. When does a stain on colored undergarments become halachically a flow and make her into a nidda? I heard in the name of Rabbi Feinstein that the amount is the size of a half dollar bill. is this correct?