Though modern slavery exists, why is slavery still associated with skin complexion?

There is a master-slave relationship in slavery in which the master is superior while the slave is inferior. So how do you discriminate between the two? Skin complexion is one characteristic that is easily discernable.

I believe that our society refuses to let go of the past, some people believe that slavery was not a bad thing (i.e. KKK), some people feel guilty, as if it were their fault that it happened in the first place, other people feel that they are owed something because their ancestors were slaves. I think that all of these people need a swift kick in the pants to help them get over it lol. My ancestors were scottish, if you ever watched the move Rob Roy....or Braveheart, you know what the English did, yet you don't hear scottish people ranting and raving about equal rights because they were almost bred out of existance....So in short...people are living in the past and refuse to let go..there's my opinion, thanks for shopping, have a nice day :-)

I think that modern slavery is against the minority with a different skin color than the majority because many times a majority of people view them selves as a greater importance becsaue of their number, and look down upon a majority, very often one with a different skin color.