While in our day, those born with white skin spend endless hours in the sun trying to darken their skin, the culture of the Nephites was different. At least among the righteous, white skin was preferred because of its symbolism. The curse of dark skin had come upon the Lamanites because of their wicked fathers (2 Nephi 5:21), but by A.D. 13, it was miraculously removed—seemingly in just one year. Both parents and children actually had the color of their skin changed. Yet, many have applied this scripture incorrectly. They assume because this miracle happened in one generation of Lamanites that it should happen in every generation of Lamanites. The Book of Mormon does not support this universal application.
Consider the people of Ammon. They were incredibly righteous, none of them ever fell away, and they endured to the end. Yet never once does Mormon mention that their skin became white. He did say that the curse of God did no more follow them (Alma 23:18), but this meant that they were no longer cut off from the presence of God by their rebellion. The same is true of many other groups of righteous Lamanites. Speaking of the last days, when the Lamanites would turn again to the Lord, Nephi said, their scales of darkness shall begin to fall from their eyes; and many generations shall not pass away among them, save they shall be a pure and a delightsome people (2 Nephi 30:6, italics added). Nephi did not say that the darkness should be removed from their skin but from their eyes, as they were to be illuminated by the light of the gospel.
The Lord can bless any generation of Lamanites with whatever blessing he chooses. However, it seems evident that what happened in this particular year, to this particular group of Lamanites, was part of the many signs which were given to the people. The Lord was going to leave the wicked without excuse prior to their impending destruction. Accordingly, this miraculous change in skin color was given as a sign to the people, for there began to be greater signs and greater miracles wrought among the people (3 Nephi 1:4).
Joseph Fielding Smith
“The dark skin was the sign of the curse. The curse was the withdrawal of the Spirit of the Lord…The dark skin of those who have come into the Church is no longer to be considered a sign of the curse…these converts are delightsome and have the Spirit of the Lord.” (Answers To Gospel Questions, vol 3, p. 122-3)