The Kollel writes-
>>(j) Support (Beraisa): One who shaves the length of a scissors blade on Shabbos is liable - this is two hairs;
(k) R. Eliezer says, he is liable for one hair.
1. Chachamim admit, one is liable for removing a single white hair among black ones (because this is important to him);
2. Men are forbidden to do so even on a weekday - "V'Lo Yilbash Gever Simlas Ishah" (do not adorn yourself like a woman). (Rambam - he is lashed even for one hair; Ra'avad - it is forbidden only if he removes enough to make a recognizable difference; Ritva - our Tana holds that this is only forbidden mid'Rabanan on account of "Simlas Ishah").<<
This is fine. However, shouldn't he also be chayav for doing it on shabbat because of "Borer" (that you aren't allowed to separate the bad from good on shabbat) - here is is taking a white hair from the groups of black. This is assur also!
The Melachah of Borer is considered a Tikun in the remaining good, desirable objects, in that the remainder becomes more usable since the bad was removed. See Or Same'ach (Hilchos Shabbos 8:11), who says that for this reason there is Borer only when the remainder is used together; in contrast, there is no Borer for Kelim (utensils) which are used individually. (Not all Poskim accept this reasoning.)
Therefore, in the cae of hairs, one is not improving the remaining black hairs for they are not for use anyway. One merely wants to do away with the white hair. Although it is among the black, it still is not Borer.
It is also possible that since each hair is connected individually to the body, it is not considered a mixture (Ta'aruves) which is neccesary to have before there is a problem of Borer. This is my suggestion, but I would not rely on it l'Halachah.