Grey jeans come into their own in that stretch between seasons when black feels too heavy and blue feels like you haven't thought about it at all. Early fog, the first proper coat of the year, that bit of autumn or spring when the light is flat and you want something that looks considered without trying very hard. Inside leg 34 inches, I need 35, and in a regular fit that half inch shortfall shows up exactly where it always does, sitting half an inch above my ankle bone instead of at it. Grey shows every crease and every centimetre of hem more than blue does, so the fit problem is harder to hide. Worth knowing before you fall for the colour.
Where grey jeans go wrong is anywhere you need the jean to disappear into the background. Funerals, obviously, where even a soft grey reads as "tried but didn't want to commit." Also anywhere properly smart, because grey denim sits in a strange middle ground, more dressed than blue, never quite as dressed as trousers. The difference between a good pair and a disappointing one is mostly in the dye and the weight. Cheap grey denim often has a flat, slightly chalky finish that looks like it's been washed forty times before you've bought it, and it fades unevenly at the knee and seat within a few wears. A good pair has a bit of depth to the grey, closer to charcoal or stone than to pale ash, and the fabric has enough weight that it holds its shape at the knee instead of bagging by lunchtime. Check the inside seams and the button stitching while you're still in the shop, loose threads there are a decent predictor of what the hem will look like in a month. Rivets and the button should feel properly set, not like they'll work loose in the wash.
Grey jeans clash with anything beige or stone coloured worn alongside them, the two neutrals just compete and look like an accident rather than a decision. They also fight with very pale grey or silver knitwear, which flattens the whole outfit into one tone with no contrast. What works without any effort is a white shirt, a navy jumper, black boots, the sort of things already hanging in most people's wardrobes. A denim jacket over grey jeans looks deliberate rather than clashing, which surprised me the first time I tried it.
A good grey jean is quietly more useful than people give it credit for, and considerably harder to get right than blue.