The thing nobody tells you in the shop is what happens to the inner thigh seam after about six weeks of actual wear. Cheap denim starts to go soft and pale right along that seam first, a faint white line that no amount of washing fixes, while a heavier denim with proper weight to it holds its colour and shape in exactly that spot for years. You only find this out by living in them, not by trying them on under shop lighting for ninety seconds.
They are the wrong choice for anything that needs you to sit cross legged on a low chair at work, anything with a strict dress code, and anything in weather that drops below about fifteen degrees unless you enjoy goosebumps as a style choice. They are also a poor choice if your thighs rub when you walk, because cheap denim with no stretch will chafe within an hour and good denim with a touch of elastane will still chafe eventually, just later. For colour, a mid blue wash with a bit of fade is the easiest thing to live with, it goes with almost everything and doesn't show chalky marks the way very pale bleached washes do after a few wears. Dark indigo is smarter and more forgiving if you want to wear them with something slightly dressed up on top, but it shows white thread and stray fluff more than mid blue does. Black denim shorts are the least forgiving of the three, they fade to grey unevenly unless you wash them inside out on cold, and the fading never looks deliberate the way it does on blue denim.
Before you buy, check the rise with your hand flat against the front, not just a tape measure held up to the label, because a two inch difference in rise changes how the shorts sit on your stomach far more than it changes the length. Check the weight of the fabric between your fingers, thin and slightly see through in bright light means it will go baggy at the back within a month. Check the pocket lining doesn't hang lower than the hem, which happens more than it should and looks terrible the first time you reach for your phone. Check the stitching on any raw or frayed hem is actually finished underneath, otherwise the fraying will keep going well past where you want it to stop.
As for what they clash with, avoid pairing them with anything else in heavy denim unless the washes are clearly different shades, a matching denim jacket and shorts in the same blue reads like you got dressed in the dark. They also sit badly under a long, structured coat, which makes the whole outfit look like it forgot to finish getting dressed.