All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If a woman has a regular and steady menstrual period then she has to follow it even if the color of her menses is different. It is considered as menses as long as it comes during her regular menstrual period regardless of the color of menses whether it is red or red blackish or brown. Basically, the color of menses is blackish. The Prophet said: "The menses blood is black and it is known." [An-Nasaa'ee and Abu Daawood]
Allaah Knows best.