A kesem that is found on a colored piece of clothing is tahor. What about someone with dark skin (they are of some ethnicity/race that has darker skin), could the same idea be applied in such a case that the cesem would also be tahor?
Chut Shani (Rav Nissim Karelitz, Niddah, Shaar Ha-Zion 168) writes that this is a matter of doubt.
However, it stands to reason that a kesem will be tamei irrespective of skin color. Even among regular European Jews, skin colors range from bright to darker, and this is certainly true of Sephardi Jews. Nobody has ever suggested that the halachah of kesamim on the body will not apply to Sephardi or Middle-Eastern Jews (see also Mishnah, Nega’im 2:1, where skin color is noted as being not black and not white, but in between).
Therefore, it appears that the halachah of colored clothing applies specifically to kesamim on clothing, and not to the body.