The conventions about footwear are that you aren't supposed to wear brown boots with certain choices of clothing, or with certain colors, or without a matching belt and yadda yadda yadda. 

Who says so? Who comes up with this junk? Does it really matter what you wear your awesome brown boots with?

Like are the literal fashion police going to arrest you?!

Of course not. There are no actual fashion least that we're aware of. 

Like other arbitrary "rules" about what's right or wrong about what you wear, the "rules" about wearing brown footwear came about a long time ago but haven't been cast on the ash heap of history where they probably belong. 

Brown Boots And Black Ties Come From British Fashions

The quick version of where the silly rules about brown boots come from is English aristocracy. 

The way "fashions" in basically anything work is that rich people start doing something. Slightly less rich people do the same thing to feel richer. Slightly less rich people than that do likewise and all the way down until middle class and working folks start doing it to please their bosses. 

Ever wondered why lobster is so expensive, despite lobster basically being an underwater cockroach? Same process. 

During the 19th century, the British aristocracy started to dress in a more conservative fashion, until the first modern dinner jackets started showing up in the 1860s. Around this same time period, there was a social movement called the Great Male Renunciation which rejected flamboyant dress and styles for plain, functional clothing.

What we recognize as a suit jacket today was actually the outdoor jacket of the 1800s and 1900s. Today's tweed blazer was yesterday's Carhartt. 

Anything but simple, earthy colors was considered bad. Formal events demanded the most conservative of color schemes and black and white (the favorite of several British royals) became "the" colors to wear if the occasion was fancy. 

The way things worked in America in those days was that anything fashionable in London eventually became the fashion in New York and elsewhere. For instance, we call a black dinner jacket a "tuxedo" because the fashion caught on heavily in Tuxedo Park, NY, a wealthy NYC suburb.  

That's where we got these ideas about brown boots and shoes. Black and white are the colors of formality and being serious, and anything less is too proletarian for the office. These rigid "rules" about how to dress persisted until well into the 20th century, and some people are still holding onto them for dear life. 

Brown Boots Get A Boost From The 70s

While there isn't really a date one can use, it's generally understood that the color brown and other earth tones came roaring back in a big way in the 1970s. The 1950s and 1960s were culturally ripe, with waves of countercultures emerging and influencing fashions. 

Granted, some of them were tragic. Corduroy was acceptable to wear in public. People wore bell bottoms without being questioned. However, that's also when brown leather boots started to catch on as being fashionable. 

Obviously this is all silly. "Fashion" is an arbitrary construct; it has nothing to do with anything practical (like wearing a suit...indoors) and makes no sense whatsoever.