Can you tell if you're black or white?

Last winter a story surfaced about "black" and "white" twins. As you can see by the picture the main difference is in skin color, though genetically full sisters (fraternal twins), one twin has the complexion typical of a northern European, while the other is darker skinned. Contrary to the news reports the darker skinned twin does not seem to exhibit the modal complexion of sub-Saharan Africans, rather, she is several shades lighter. In fact, the photo suggests that she is about the same color as her parents, who are both genetically 1/2 European and 1/2 black.^* Seeing as how adults are generally darker than they were as infants it is not unreasonable to assume that "black twin" will be darker than either of her parents by adulthood, just as her sister is lighter than either of them as a child (and will likely remain so). In any case, for the exposition below I will assume that one twin is "black" and the other is "white." In light of my recent posts about skin color, I thought this was an appropriate time to use this old story (which according to google remains popular) as an exemplar of Mendelian genetics beyond the single locus, that is, an exploration of variation on traits generated by a confluence of loci. The recent evidence suggests that about 5 loci are responsible for about 90% of the difference of average effect in regards to phenotype (i.e., skin reflectance) for skin color between populations (these are loci of "large effect").