@nist, the root cause was/is dissimilarity:  the difference in perception, apprehension, lifestyle, culture, anything that's incongruous.

Color was possibly one of the stereotypical methods of identifying someone as 'different', which is not necessarily the case anymore; especially among the mixed societies. I believe it's the 'attitude' that governs the characterization in modern days.  

You'll probably get an 'F' on your paper if you include my rants; but I hope this makes sense if you wear a 'generic' hat and chew over!

~@~ 

PS: This link might help you: http://en.wikipedia.org/wiki/Racism_in_the_United_States