The Balance of Summer and City Cool.
Brown never in town. An expression dating back to the Victorian into Edwardian era pertaining to the mix and match of country versus city attire and not confusing the two; essentially brown (earthy) shoes for the country, black for the city.
This, in turn, extended to the fabric and colour of apparel worn, with black in the City conservatively teamed with a light wool navy or grey suit (not forgetting the pinstripes) and a white or light blue shirt, with possibly a little personality offered by the tie and pocket square. Hence the traditional look of the city, banker gent.
The country of course was about your Tweeds.
Today with the City encompassing more than just banking and possibly the desire to not look like a banker, it is no longer always quite so rigid; equally (while not withstanding the consistently on form women and younger gents who have have upped the style stakes) not everyone working in the city necessarily has a brown shoe wearing country estate these days..
All that said, even with a more relaxed attitude to uniform certain sartorial rules should remain observed; there will be occasions and locations where the code will not deviate from black & navy though, and more so today, there are many environments where there is a little more scope for colour.
This is a great help for hot, sunny, summer days. Offering the chance to break out of the shadows and dress for the weather, while remaining suitably formal and respectable. A much more sartorially dapper approach than simply going without a tie (questionable) or a pair of suit trousers without the suit jacket (unforgivable).
When embracing something other than black this needs to be done with consideration and awareness, being fully aware of your colour combinations and allowing colours to compliment (say the hue) or contrast (though not garishly).
It starts of course with your suit and shoes, from which you build.
When choosing your tie, pocket square and socks, trying hard to match colours and patterns will look too contrived, while extravagant contrasts will look too busy and cluttered. Subtlety and an informed abandon is key - knowing precisely the look you are presenting. 'Sprezzatura'.
However, if ever in doubt, play it safe rather than make a mistake.
With all the above in mind, avoid at all costs a mix of black and brown. If wearing black (shoes) a belt should be black (if wearing a suit with loops that is) and complemented with a black (preferably leather) bag (navy or grey if done well).
If in brown you have a little more scope for colour, though again, avoid black, and be careful not to display a mix of wildly different browns. All this even goes down to the colour of your watch strap if this is not a bracelet. This way you are making a concerted distinction between the two camps.
Whichever direction you follow, be relaxed, don't be afraid to show a little personality and thus... set yourself apart.