I though that first too, that later she will change her clothes back to normal. But in the last Chapter of Hellsing ("30 years later") Seras still have dark uniform in the manga. (because of the artstyle we can not be sure about the color) And her glove (the right one, cuz she doesn't had a left hand) turned from brown to white, that means she isn't turned to red ONLY because of the blood, it is some symbolic meaning like for Alucard's suit (my coat is painted to red from the blood of my enemies)