Why is skin color used more often than hair or eye color or body proportions in racial classifications?

There are several ways racial classifications could be maintained.  Skin color is the predominant manner of classification for three reasons.  First, it is the most prevalent characteristic of people.  Skin is the largest organ of the body and covers the entire surface of a person thereby making the color an easily identifiable marking. 

Secondly, skin color is the most difficult to alter.  Hair can be dyed quickly, contacts can be worn to alter eye color and physical proportions can be disguised or changed.  It is possible to change skin tone but it is a difficult and painful process.  Disguising it with clothing is an option, but due to the amount of skin requiring concealment it is not usually feasible.

Finally, skin color is a characteristic most ubiquitous to a racial group.  Eye and hair color vary at a greater rate than skin color.  This makes skin a more optimal marker.  Other markers can include eye shape, muscle density and body proportions but they are harder to spot at a distance and are subject to greater fluctuation throughout a population.