The thing to know before you buy a cotton jacket is that it has no stretch and no drape to speak of, so it will show you exactly where your shoulders are. A knit jacket forgives a bit of slouch. Cotton doesn't. Try it on standing the way you actually stand, not the way you stand for three seconds in a changing room mirror, and if the shoulder seam sits even half an inch off the edge of your own shoulder, that's the jacket telling you, not you telling the jacket.
Where it falls down is weather and warmth, obviously, but also structure. A proper cotton jacket, the kind with a bit of body to it, is a three season piece at best, it does nothing against real wind and nothing against real cold, and if you're after something to see you through an actual Edinburgh winter you want wool or you want to be lying to yourself. It's also not the jacket for anything that needs to move with you. I wore one to carry boxes up four flights once and the back seam across the shoulder blades did not thank me for it, there's a specific pulling sound a cotton weave makes when you've asked too much of it. Where it is the right choice is everything in between, the months where you need a layer that holds its shape rather than one that just hangs there. In the shop, check the weight of it first, hold the fabric between two fingers, a cotton jacket that feels like a shirt will act like one, no structure at the collar, no weight in the hem. Check the lining, or the lack of one, because an unlined cotton jacket creases the second you sit down in a car. And check where the hem lands relative to your hip, because a cotton jacket that stops exactly at the widest point does you no favours at all, you want it a finger's width above or a good two inches below, nothing in between.
For colour, the ones that earn their place over a full year are the ones that already look slightly worn in, a faded khaki, a washed stone, an off white that's clearly never aimed for actual white. They sit happily over a plain jumper and jeans, over a cotton dress in the warmer months, over nothing more complicated than a t shirt on a Tuesday. True white shows every coffee drip within a week, and a very stiff navy can look like a school uniform if you're not careful with what's underneath it.
The mistake is buying it a size up for comfort. Cotton doesn't give you that comfort back, it just gives you shoulders two inches too wide and a jacket that looks borrowed.