It used to be that people went about after labor day wearing white all the time. This was an okay practice during the fall and it actually made the beautiful colors of the leaves stand out so much better. But when the snow fell, people lost each other really easily.
For instance, two people were out hunting some deer one winter before this rule was made; one had on white clothes, the other had on whatever. The decided to split up to cover more ground. Several hours later, the guy wearing multi-colored clothing was getting very tired. He'd been out all day with only some cold biscuits to eat, there was something sticking into his butt from the tree branch on which he sat, and his eyelids were getting very heavy. Suddenly, out of the corner of his eye, he saw something move. He turned and squinted, but couldn't really make it out. But it looked alive and deerish, so he shot. Jumping off the tree, he ran over to his quarry to drag it home...only to find out he'd shot his partner in the leg, not a deer.
The moral of that story, dear children, is to never shoot at something when you're tired.
But anyway, people were tired of losing each other in the winter, so they made the rule of no white after Halloween. After a while, they began to realize just how ridiculous it was to wear white in the fall because it rained a lot then and got EVERYTHING muddy. So by the end of the day, even if you had started out wearing white, you came home wearing brown - or even black, if there had been a real downpour. To save their clothes from this disastrous end, people came up with the rule not to wear white after labor day.
This was at the time when people didn't live in places like Arizona or Jamaica where the weather's hardly bad. And it was before global warming. Now there's hardly any snow and people live all over the place, so the rule isn't so applicable anymore.
Historical facts everybody knows - and maybe a bit of my own imagination