The admonition to “turn ye unto the Lord” is that the unbelievers should come to a belief in the Savior prior to the time of his coming. Since they will be in great anguish as unbelievers if they are in that state when he comes, logically they should become believers before that time.
Cultural: The set of terms used here for a state of repentance is: “spotless, pure, fair, and white, having been cleansed.” The cleansing gives us the context of “white” in this case. The imagery is the basic Biblical imagery of becoming clean and spotless, therefore “white.” This is not necessarily the same imagery as in the “white and delightsome” pairing in Nephite vocabulary, although the presence of the term “white” in that formulation may come from the same western tradition as this reference to cleanliness.