I agree that we should be careful with people who might be new converts or poor and therefore might not own or be able to afford white shirts. However the way this is worded it seems to me that those are the two exceptions. If you are new or cannot afford a white shirt you might get a pass for a while. Maybe someone can lovingly donate you one and then you can be in compliance.Bishops should use discretion when giving such guidance to young men, taking into account their financial circumstances and maturity in the Church.
When, where, and why did the policy come about for white clothes in baptism, in the temple, and on missions? So many times in the church I think that we are creating justifications for the way things are. I see this also with scriptures. We search the scriptures for verses to justify, bolster, and defend what we already do in the church. Sometimes, we take the words out of context or even twist their meaning to make our point.and a white shirt could be seen as a gentle reminder of the white clothing you wore in the baptismal font and an anticipation of the white shirt you will soon wear into the temple and onto your missions.....As President David O. McKay taught, a white shirt contributes to the sacredness of the holy sacrament.
P.S. I believe that Catholics can also feel that performing mass in Latin contributes to the sacredness. To me it seems to be code for "that is the way it is done and we like it that way."