Navy looks like the safe choice and then you get it home under a different light and it's basically black, or worse, it's got a green undertone that fights with everything else you own. Try it on near a window if you can, not under shop spotlights, because that's where navy tells the truth. It's still worth the bother though, because a proper navy does something black can't: it reads as dressed rather than dressed in a hurry, and it doesn't flatten you out the way head to toe black sometimes does.
A decent navy skirt ages well if you treat it a bit like denim rather than like a cheap print. Cold wash, turn it inside out, skip the tumble dryer unless you fancy it a size smaller by October. Over months of actual wear, cheap navy goes patchy at the seat and fades unevenly along folds before anywhere else, so check those spots after a few washes rather than waiting for a disaster. A good one keeps its depth of colour and the hem keeps its shape, which matters more on a skirt than on almost anything else, since a wonky hem is the first thing anyone notices from behind.
On shade, the easiest navy to live with is a true, slightly dusty navy rather than anything too bright or too inky. Bright navy can look a bit uniform, like you're about to direct traffic, and very dark navy can tip into looking like bad black in certain lighting, which rather defeats the point of choosing navy in the first place. A midtone, slightly worn in navy pairs with more of what's already hanging in your cupboard, white, grey, tan, even black at a push, without you having to think hard about it on a Tuesday morning.
Before you buy, do the usual hand checks. Scrunch a section in your fist and see how many creases stay behind, that tells you roughly how it'll look after a few hours sitting down. Hold it up to the window if it's a lighter weight fabric, because a skirt that goes see through under daylight is a skirt that needs a slip, no way round it. Check the waistband construction too, a flat waistband on anything with a bit of stretch tends to roll over itself by lunchtime, which is a small thing until you're tugging at it on a train platform.
The one mistake worth avoiding is buying navy purely because it felt like the grown up, foolproof option without checking it actually suits your skin tone, because navy next to the wrong undertone can leave you looking a bit washed out rather than polished.