Must prayers performed in clothes with slight menstrual blood spots that were not noticed until days later be repeated?

You are not required to repeat the prayers you performed while wearing ritually impure clothes, as you were unaware of the presence of the impurity. If you mean that the color of blood remained after washing the clothes, then the persistence of the color of menstrual blood after washing it does not cause any harm, based on the Prophet's (peace be upon him) saying to Khawlah: "Water is sufficient for you, and its trace will not harm you." The majority of jurists hold the view that if the removal of the color of impurity is difficult, then the spot becomes pure through washing, and the persistence of the color does not cause any harm.