I must say I'm puzzled first at the idea that there was a standard and then gobsmacked that it appears to be true judging on the replies above. I almost harumphed at the idea that blokes wore black / white only. Y'learn summat new everyday.
I myself wear all sorts of colours. I just treated myself to some socks with Monday / Tuesday / Wednesday etc on them. I will have enormous fun at some point
deliberately having different days on different feet. It's a full life.
Also, in the summer I never wear socks.