when my boys were in mesivtah they were permitted to wear only white shirts, but the boys in the yeshiva gedolah were permitted to wear colors as long as they (the colors) were “suitable business attire” (i.e., what Jothar referred to as Slobakain “conservative elegance”). The reasoning of the Hanhalah of the yeshiva was as follows – the yeshiva bochurim were taught that they must treat yeshiva with no less kavod than they would a business meeting, and in fact were encouraged, though not required, to also wear neckties to the beis medresh, i.e., like men who wear a suit to the office but remove the jacket, but still wear a tie and dress shirt throughout the day – the rationale made quiet an impression on visitors who would visit the yeshiva and see 100 guys in ties learning – it made an impression on the boys, too, they felt important and choshuv
the high school boys wearing white was a bit of a different, albeit related, rational – the rosh mesivta was raised in a strict European home where proper manners were emphasized, not only kavod for people but also table manners, etc – the mesivtah had a strict dress code – shirts tucked in, shoes polished – they did not care about designer labels but they wanted the boys to appear clean and well kept and to feel that they, too, were important – so the rosh mesivtah instituted “white shirts only” because he felt that they boys would be more careful to not stain a white shirt in the dining room because it would show more easily, and the high school rebbeim would send boys back to their dorm rooms to change if their shirt was spotted – by extension, the rosh mesivtah thought that would improve dining room manners since careful eating would be required to keep shirts clean – it worked, one would join the boys for Shabbos or a week dinner and one felt as if (quality of the food aside) one was at a business lunch the boys ate so nicely
the white shirt rule was suspended for the purim seudah for obvious reasons