The thing nobody mentions when you buy a pair of blue jeans is how the colour moves before the shape does. Mine always fade first at the bend of the knee and along the inside of the thigh where they rub against each other, long before the waistband gives up on me, and that uneven fade is actually the tell of a decent dye job rather than a flaw. The ones that go patchy and grey all over after four washes were never going to last anyway.
Two blues rarely want to sit next to each other, so a mid wash jean worn with a similarly mid blue top just reads as a mistake rather than a styling choice, unless you genuinely commit to full denim on denim and own it with conviction. Getting the fit right matters more than the wash, and most of the actual problems live at the back, not the front. A waistband that gapes when you sit down, pockets that flare out like little wings, a rise that looks fine standing at the mirror and then digs in the second you're in a car seat for twenty minutes, these are the things that only show up once you're living in them rather than posing in the changing room. If you're between sizes, go up rather than down, because denim with any stretch in it will relax and drop within a day of wear, and a jean that's snug on day one is painfully tight by evening.
What separates a good pair from a sad one is mostly in things you can't see on the hanger. Heavier cotton with a bit of structure holds its shape through a day of sitting and standing far better than the thin, almost papery stretch denim that bags at the knee by lunchtime and never quite bounces back. Decent stitching sits flat and even along the inseam rather than puckering, and the rivets and button should feel properly set rather than like they'll pop with one enthusiastic squat. Cheap denim also tends to smell faintly chemical when it's new, which is never a good sign of what it'll do to pale furniture or white trainers in the first few washes.
Before you commit, do the unglamorous checks in the shop rather than at home with the tags still on. Sit down fully and see where the waistband lands on your back. Walk a few steps and watch whether the fly gapes. Check the inside pocket lining hasn't been cut from some flimsy scrap that'll tear the first time you shove keys in, and run a finger along the inseam for any pulling or loose thread, because that seam takes more strain than any other part of the jean.