white is not a skin color. white is not about physical appearance or physical characteristics. while skin color highly correlates to whether or not you are considered white, it is not about the pigmentation in your skin. there are people of color that are paler than people who are considered white.
white is not a biological category. white is not about genetic descent. when a person of color has a child with a white person, their children may or may not be white or colored. more to the point, this same child in one culture may be considered white and in another culture considered colored in others.
white is not universal or even global. a person can be white in one country (like mexico or brazil), and considered colored in another (like england or the united states). within the united states, people of mediterranean descent can be white in one part of the country and considered colored in others.
white as a social construct, comes with perceptions about what it means to be white, how to treat white people, how white people expect to be treated, benefits and privileges given to white, who is or isn't white, where white people live, what white people are like, what white people eat, etc. white becomes the touchstone construct for what it means to be not white, how to treat not white people, how not white people expect to be treated, benefits and privileges not given to not white, who is or isn't not white, where not white people live, what not white people are like, what not white people eat, etc.
white as a socially constructed racial characteristic is culturally normative. that is to say there are cultural norms around white. these are cultural norms that whites buy into without much examination, and that not whites also buy into, but with some resistance and friction. while it is tempting to call these cultural norms "white culture", that isn't correct or useful. this collection of cultural norms is a kind of "meta culture" - a "culture" that is shared by/included in many cultures.
these cultural norms are what define white.