Once again the possibility arises that Nephi is describing a skin color, and attaching value to it. This verse is probably one of the most difficult to exegesize into a more palatable modern interpretation, as the "white, and exceedingly fair and beautiful" certainly all sound like references to physical appearance.
However, the analysis that the white/dark dichotomy applies to righteousness also fits here, but only as a precursor to the verse which follows. It could not be said in any good conscience that the Gentiles who conquered the Americas were virtuous. It also cannot be said that they were overly white, as skin tones go. Nor, of course, would the genetic stock from Israel produce the Northern European whiteness of skin.
The white and beautiful are still best seen as indicative of receptiveness to the Lord. In this verse, the function is to set up the next important aspect of the invaders - that they were to be the means whereby the gospel would be returned to Nephi's descendants.