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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The presence of the color of menstrual blood on the garment after washing it is harmless. The Prophet said to Khawlah, may Allaah be pleased with her: "Water is sufficient (for washing) and the remaining color does not harm you." [Al-Bukhari]
Allaah Knows best.