Matthew 5:36 (King James Bible) because thou canst not make one hair white or black
Here the 1874 RLDS edition changed the or to nor. When preceded by the negative not, either conjunct is possible in standard English. In this particular case, the 1908 RLDS edition restored the original or (probably by reference to 𝓟). For further discussion of the acceptability of either or or nor in this kind of negative expression, see under Alma 26:11.
It should also be noted here that the order of the conjuncts white and black differs: “black or white” in the Book of Mormon text, but “white or black” in the King James Bible (and in the original Greek). It is possible that the Book of Mormon order is an error that occurred during the dictation of the text by Joseph Smith. (𝓞 itself probably had the order “black or white” since both the printer’s manuscript and the 1830 edition read this way.) On the other hand, the listing of black first may be characteristic of Nephite culture or represent the common phraseology of the language itself, as found elsewhere in the text:
Given this minor evidence, there seems to be no strong motivation to emend the word order in 3 Nephi 12:36.
Summary: Maintain in 3 Nephi 12:36 the conjunctive or, the reading of the earliest extant text (“thou canst not make one hair black or white”); also maintain the conjunctive order of the adjectives, with black before white, the reading of both 𝓟 and the 1830 edition; that order also occurs elsewhere in the Book of Mormon (in 2 Nephi 26:33).