I didn't realize at the time I read Harriet the Spy why the boy with the purple socks struck me, but I know now. You don't often run into a story that hints at one of the minor characters working around a problem with prosopagnosia in the family. I've never been able to see faces in a crowd, and I've always looked for other markers to find my people, their clothing du jour being a big one, although that turns out to be a bit tricky sometimes when I turn back from looking at something on a shelf and follow the wrong person around because they're wearing similar clothing and have a similar shape from behind. So purple socks made a lot of sense to me when I read that, while others probably found it eccentric and a weird thing to say.
My sox thing morphed into a super sox thing during my retailing years (I supplied everyone I knew with employee discounted clearance), and a decade later pretty much flung myself off the sox cliff when the rest of the world finally caught up with the novelty mismating thing. I take my sox very seriously.
Other people do too, apparently.