The real tell with denim shorts isn't how they look in the changing room mirror, it's what happens to the inner thigh seam after about the eighth wear. Cheap denim starts to fray there within a month, a scratchy little seam that catches against your leg every time you sit down. Good denim, the kind with a proper felled seam rather than a single overlocked stitch, just softens instead. You only find this out by living in them, which is annoying, because by then you've already decided whether you like the cut.
They're the wrong choice for anything involving a bike saddle, a short walk becomes a different story, and for anyone who spends the day on their feet behind a counter or a till, since the fabric digs in at the top of the leg in a way trousers never do. I'd also think twice before wearing pale or white denim shorts anywhere near a picnic rug or a day with small children, for reasons I don't need to explain to anyone who's tried. Cheap stretch denim is the other trap. It gives you shape for the first wear, then goes baggy at the back by the third wash and never quite recovers, which is why the ones with a bit of real cotton in the mix, rather than almost all elastane, tend to earn their keep longer.
Mid blue, a proper worn in looking wash rather than anything too dark or too bleached, is the easiest shade to live with, it goes with almost everything already in your bag and doesn't show marks the way pale denim does. Black denim shorts look sharp for about six washes and then fade to a flat grey that reads as tired rather than deliberate, so only buy them if you're happy to retire them early. In the shop, check the weight of the fabric by scrunching a handful in your fist, if it springs back slowly it'll hold its shape, if it stays creased it'll bag at the knee by lunchtime. Check the rise too, sitting down on the shop floor stool tells you more than standing in front of the mirror ever will.
Keep them away from anything with a lot going on below the waist, a busy printed top is plenty once you've got denim doing its own thing underneath, and steer clear of pairing dark denim shorts with dark tights in cooler weather, it reads as odd rather than considered.