What the Book of Mormon says about ...

Skin color

God changed the Lamanite's skin from fair and delightsome [1] to black -- to make them less appealing to the Nephites. This is why Native Americans (who are the descendants of the Lamanites) have dark skin.

And he [God] had caused the cursing to come upon them [the Lamanites] ... wherefore, as they were white, and exceedingly fair and delightsome, that they might not be enticing unto my people the Lord God did cause a skin of blackness to come upon them. 2 Nephi 5:21

God said that any Nephite that mixed their seed with the dark-skinned Lamanites would receive the same cursing from God. (Their skin would be turned dark, too.)

And cursed shall be the seed of him that mixeth with their seed; for they shall be cursed even with the same cursing. And the Lord spake it, and it was done. 2 Nephi 5:23

If people dwindle in unbelief, their skin turns dark and they become a loathsome and filthy people, full of idleness and abominations.

And the angel said unto me: Behold these shall dwindle in unbelief. And it came to pass that I beheld, after they had dwindled in unbelief they became a dark, and loathsome, and a filthy people, full of idleness and all manner of abominations. 1 Nephi 12:22-23

The curse of dark skin is reversible, though. If dark-skinned people behave themselves, their skin will be lightened and they will become white and delightsome again. (This may take a few generations, however.)

And then shall they rejoice; for they shall know that it is a blessing unto them from the hand of God; and 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 white pure and delightsome people. 2 Nephi 30:6

The descendants of Cain were black. [3]

And Enoch also beheld the residue of the people which were the sons of Adam; and they were a mixture of all the seed of Adam save it was the seed of Cain, for the seed of Cain were black, and had not place among them. Moses 7:22