If he performs istibrā from semen by urinating, the process of istibrā is correct, even if blood was discharged after that. He can then perform ghusl and ensure tahir water reaches all parts of his body. Similarly, if the blood stops for a short period of time and he performed ghusl during that time, the ghusl is correct even if blood is discharged after that.
It is permissible to wear red clothes, but it could be in some cases unfamiliar to the society, so whoever wears red clothes could subject himself to embarrassment or humiliation.
