If you're holding a pair of ballet flats in the changing room, bend the sole in half before anything else. If it creases into a hard fold and stays there, put them back. A flat that can't flex with your foot is going to rub your heel raw by lunchtime, and no amount of breaking in fixes a sole that was never built to move.
These aren't the right shoe for distance walking or anything with a proper incline, there's rarely enough underfoot to cushion a long day on pavements, and the thin leather or faux leather uppers can let damp straight through in a downpour. I learned that one on a wet Tuesday with cardboard-thin soles and socks that were soaked by the time I got where I was going. They're also not brilliant if you need arch support, most have none at all, so if your feet ache by mid afternoon in flat shoes generally, this style won't change that.
For colours, black and a mid tone nude are the ones that earn their space, they go under trousers and dresses without a second thought and don't show scuffs the way pale pink or white do within a fortnight. A soft tan is nearly as forgiving. Bright colours and patent finishes look lovely in the shop and then show every scrape on the toe within a few wears, so go in with your eyes open if you choose one.
The difference between a flat that lasts and one that's bin fodder by spring usually comes down to the inside, not the outside. A proper leather lining breathes and moulds to your foot, a synthetic one stays rigid and tends to make your foot sweat and slide, which is exactly how blisters start. Check the stitching round the toe seam too, loose or uneven threads there are usually the first thing to go. A stacked or reinforced heel edge will survive pavement longer than a flimsy one that wears to an angle within a season.
Before you buy, press your thumb into the insole and see if there's any give at all, a completely flat, hard insole is a sign of corners cut. Check the elastic if there is any at the sides, it should hold the shoe close without pinching, and have a proper look at the toe box with your foot in rather than just glancing down, because narrow ones that look fine standing still often pinch the moment you walk.
Wipe them over with a damp cloth after a wet day rather than leaving them to dry stiff near a radiator.