Dear 100 Hour Board,
In a primary lesson last Sunday, we were talking about baptism, and the fact that we always get baptized in white clothes came up. Obviously, in the earliest days of the Church this was not the case-Joseph Smith and Oliver Cowdery didn't go find all white clothes to change into before they baptized each other, nor was that part of John the Baptist's instructions, so when did wearing white clothes become part of the ordinance? And is it something that is just a tradition, or would the ordinance be considered invalid if someone was not wearing all white?
-Don't remember covering this in the MTC...
You are correct to assume that wearing white clothes hasn't always been part of baptisms in the church. Pinning down when the transition to white clothing began was quite difficult however. I spent several hours searching through google, the church website, and a linguistic database of every general conference talk. Individual terms such as baptism, white, or clothing gave far too many results, and including all three terms didn't yield any search results very far back. The earliest mention of white baptismal clothing I could find in a church publication was 1959, so it likely happened sometime before then.
However, I personally know someone who was baptized in clothing that wasn't completely white during the 1980's. This person was baptized in a white pajama dress with pink polka dots because there wasn't any baptismal clothing her size in her branch or anywhere nearby. I think this illustrates that white clothing is important, but can be substituted if not available. My guess is that white clothing became standard for baptismal services as the church began to make and distribute clothing white baptismal clothing. This photo of Boyd K. Packer performing a baptism in Japan was from 1945 so it was likely standard before then as well. My best guess is that white clothing became standard sometime before the 1940s.
Is white clothing required for a baptism to be valid? Here's what the Handbook of Instructions says about baptismal attire: "A person who performs a baptism and a person who is baptized wear white clothing that does not appear transparent when it is wet. An endowed person wears the temple garment under this clothing while performing a baptism."
This statement is clear on what the standard is. I think that while white clothing isn't specifically mentioned in the scriptures as part of baptism, the symbolism of white clothing representing purity adds to the reverence of the ordinance. In most cases obtaining white baptismal clothing is possible so this typically is not an issue. While the statement doesn't specifically say whether or not wearing clothing that wasn't white would invalidate the ordinance, my best guess is that if clothing other than white had to be used, then it would be fine. If there's not a baptismal font available any body of water will do for a baptism. If there isn't any bread available something else could be used for the sacrament. I imagine the same principal still applies for baptisms today. The ordinance itself is what is most important, but we use certain symbols to remind us of Christ.
Sorry I wasn't able to get any hard line answers for you. White clothing in baptisms has definitely been a thing since the 1940s though. If any readers know any earlier official sources feel free to drop a correction. Hope this helps!