Brown bags used to mean one thing, a slightly apologetic tan satchel that sat in the sale rail because nobody could decide if it was smart or scruffy. That has shifted properly in the last few years. Brown has split into a whole range of finishes, tan, chocolate, caramel, a deep espresso that behaves almost like black, and the bag has gone from a backup to something people reach for first. It is a better situation than it was, mostly because brown no longer has to be the colour you settle for when black feels too much.
The shades that are easiest to live with sit at either end, a pale honey or camel, and a near black espresso brown. The camel tones lift anything dark and go through spring and summer without looking like they are trying too hard, while the deeper browns do the job of a black bag but feel warmer against camel coats, cream knits and denim. The middle ground, a very orange tan, is the one that asks more of you, it wants gold hardware, it wants warm tones nearby, and it can look dated fast next to anything cool toned. Keep brown away from black as a pairing unless the brown is genuinely near black itself, the two together usually just look like a mistake rather than a choice, and be careful with navy, which can turn a nice tan slightly muddy in low light. Grey is the safer neutral to test it against if you are unsure.
Where brown properly struggles is anywhere cold and polished is the aim, a silver grey outfit, icy pastels, anything where you want the bag to disappear rather than add warmth. It also shows weather more than black does. A pale caramel bag on a wet pavement in a Leicester car park picks up splashback within a week, and suede brown finishes are hopeless in anything but dry weather. If you are buying one bag for a long train commute through a typical British winter, brown in a matte leather or good quality faux is fine, brown suede is not.
If you have none at all, start with a mid tan in a structured shape, something with a handle rather than only a strap, in a leather or leather look that is not glossy and not suede. That one shade works with denim, with black, with grey, with most of the greens and creams that turn up every autumn, and it does not demand a particular season. It will not replace black for evening, and it will never quite disappear the way black does, but for daytime it covers more ground than people expect from a single colour.