Skin comes in an infinite array of colors. It is just our human tendency to simplify and organize that groups skin tones into white, black, brown and so on.
However, there is nothing about skin tone that makes people similar or different, besides the skin tone itself.
In fact, no one really knows what race is. If it is some kind of systematic division of humanity into physiological groups, then there are thousands of different possibilities based on genetic analysis.
If it is some kind of systematic division of humanity into socio-cultural groups, then there are thousands of different ambiguities depending on how one defines the socio-cultural group.
Race is best defined as an arbitrary categorization of people according to some set of characteristics, usually related to geographic place of historic ancestry.