They had an apocalypse to stop, but Sam and Dean still took the time to teach Cas various human things. Last week they had shown him how to do laundry. Judging by the growing pile of dirty clothes, it looked like it was time for that again. However, Sam and Dean were getting some well-deserved sleep after a late-night hunt. Luckily, Dean had, after a week of logical arguments from Sam and outright begging by Cas, taught him how to drive the Impala.
So, Cas took their dirty clothes to a nearby laundromat. He followed Sam and Dean's instructions to the letter. Or at least he thought he did. But, to his horror, when he took the clothes out of the washing machine, everything was various shades of purple. Cas immediately called Dean and said, "Dean, we have a serious problem!"
Dean answered, still half asleep, "Great, what did those flying dicks do now?" And then, as he woke up, he added, "Wait, Cas, where are you?"
He answered, "I'm at a laundromat. I saw that we were running out of clean clothes. So, I decided to take care of it as you recently taught me. But I obviously made a major error, everything is purple!" to his surprise Dean started laughing.
Cas responded, "This isn't funny, Dean. We need clothing! I often don't understand human social customs, but even I know public nudity is frowned upon!"
This only made Dean laugh harder, "Sorry Cas, A former angel of the lord forgetting to separate whites from colors; it's too funny!" Once he stopped laughing, he said, "Sometimes dye from brightly colored stuff bleeds into other clothes when you wash everything. That's why you wash colored things separately. We're heading back to Bobby's today anyway, so we'll see what he can salvage." He paused to laugh at his own joke. I'll teach you how to use Amazon Prime to order new stuff. It's not a second apocalypse. Sammy did it once too. "
Cas breathed a very human sigh of relief. "That is very reassuring." He paused as something occurred to him. "You and Sam do not have any purple clothes, or at least you did not until now. So how could purple dye bleed through?"
Dean's laughing resumed, "Thousands of years of knowledge, but you don't know that when you mix red and blue, you get purple? Is How to be an Ass Hat with Wings the only lesson in angel preschool?
Cas tried to defend himself. "Angels do not attend preschool or any other level of formal education. In any case, Father only had Gabriel announce that he had created colors. He never explained their composition."