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However, at the same time, 6 pm seems sacrosanct

It is not a rule, so much as it is a guideline. It's certainly not sacrosanct. It's meant to guide you, nothing more.

Like most guidelines, it's expected that one will apply some personal taste and common sense to its application, and not simply slavishly obey it without understanding the underlying principles upon which it's based.

The guideline might be better stated as, "More informal, lighter colored suits, jackets, shoes, etc., tend to be more for daytime wear, and more formal, darker suits, jackets, shoes, etc., tend to be more for nighttime wear, but plenty of exceptions exist."

the "rule" about wearing white or linen before Memorial Day (or Easter) is basically rubbish

And this one is more properly stated as, "Wearing white is more of a warm, light-filled, spring/summer thing, than a cold, dark, winter thing. But you should apply some personal taste and common sense to the application of this 'rule.'"

Taken that way, it's not rubbish at all.

Most of the people who cling to such rules as if they were handed down by God Himself, who burned them into stone, meant them to be eternal and unchanging, and promised eternal damnation for any transgression against them, strike me as having little understanding of the reasoning behind such the rules regarding proper dress. They need absolute rules, to which there are no exceptions, because they don't have any faith in their own judgement. Because blind obedience is so much easier than genuine comprehension.

But perhaps I'm being unfair to them. Well, I did say "most," not "all." Since there are always exceptions.