You can spot a good pair of tailored trousers from right across a room. The ones that work sit flat across the front with no pulling at the hip, break cleanly at the shoe and actually skim the leg rather than clinging or swamping it. The poor ones bunch at the crotch, go baggy at the knee by lunchtime and sit an inch too high or too low on the waist so you're forever tugging at them. That difference is almost entirely in the cut, not the price, which is the annoying bit.
Colour wise, the ones that earn their place are navy, charcoal, a mid grey and black, in that order, because they go under more things without a second thought. A soft camel or stone is lovely too but shows every mark, so it's worth thinking about where you'll actually wear it before you commit. In the shop, check the waistband sits where you want it standing up and sitting down, not just standing up, because that's where most pairs let you down. Check the pockets don't gape open when you put your hands in, check there's a proper lining at least to the knee if it's a lighter fabric, and walk a few steps rather than just looking in the mirror, because that's when you find out if the crotch is too low or the thigh is too tight to move in properly. If there's a fly front, do it up and sit down before you decide anything. A lot of trousers that look fine standing still reveal themselves the second you bend. They don't get on with anything too fussy up top. A heavily ruffled blouse or a very busy print fights the clean line of the leg and the whole outfit stops reading as put together. A plain T-shirt, a simple knit, a shirt with the sleeves pushed up, those all let the trousers do what they're meant to do. Trainers work with some cuts, particularly if there's a slight crop, but a chunky trainer under a very fluid wide leg tends to look like an accident rather than a choice.
Where they genuinely aren't the answer is anywhere you need real give, a long day on your feet, a journey with a lot of sitting and standing, anything involving bending down to pick things up repeatedly. Even a good pair of tailored trousers has less stretch than you want on those days, and no amount of clever cutting changes that. They're also not the thing for very hot weather unless the fabric is genuinely light, because a stiffer fabric with structure holds heat in a way you notice by mid afternoon. And if your day is mostly casual, trainers, a bag you live out of, nowhere formal to be, they can end up looking slightly overdressed for the rest of the outfit rather than making it better.