I think focusing on the skin colour thing is perhaps missing the bigger picture. Mankind is, by its nature, a cliquish species. We have lived in tribes for thousands of years and even today we often associate with those like us and are wary, even scared of those vastly different.
My point is: even if, through centuries of cross breeding, mankind eventually loses its indvidual races and we all end up the same race, there will be some sort of other distinguishing factor to take the place of race. Whether it be family background, education, or even (and i am taking this from Gattaca) those born from complex eugenics programs and those born naturally from unscreened parents, there will always be some way for man to seperate himself from the rest.
note: Gattaca -
http://www.imdb.com/title/tt0119177/ Just incase anyone has not seen it.