A skin color - the majority in the United States - which is apparently considered a negative quality by many, including those who are white themselves but consider themselves to be "not that white". Associated with poor dancing skills and general nerdiness. For a black person to be called white is considered highly insulting.
Essentially, if "black" were used the way "white" was, we would still be living in the 50s.
Tom (white guy): (dances poorly)
Jerry (also white): Tom, you're so white.
Doug (white guy): what are you doing on your computer?
Jeffrey (black guy): reading manga and listening to metal.
Doug: you're so white, Jeffrey.
Jeffrey: WHAT THE HELL DID YOU JUST SAY TO ME!?