Happy_LDS, actually in my ward any way its been very effective, we only seldom have a young man pass without a white shirt. Of course some don't care and probably use it as a excuse not to pass. Thats usually the exception and not the rule.
Read the church handbook of instructions. It says that it's recommendable, but not necessary. As longs as conditions allow, all priesthood bearers should dress according to church standards, which normally include a white shirt and tie. However, if a young man's family doesn't have enough money to buy a new white shirt, he should be able to use a t-shirt to pass the sacrament if that's the best he has. I see this too many times, bishops and stake presidents that don't allow youth to pass the sacrament because he doesn't have a white shirt nor tie. That is completely ABSURD!
The reason we are recommended to wear a white shirt is because if your are wearing a bright pink shirt it's possible that it could distract some people during the sacrament. It also can signify purity.
Name: Ronan
Comments: White shirts are not required but are highly recommended in countries that use the white shirt as a dress shirt. Really it should be the best you have available as pertaining to your culture. But should be as close to white as possible. Just like the person passing it. He should be as clean ass possible both inside and out.