Gilets used to mean one thing: a padded, shapeless body warmer that made you look like you were about to go beating pheasants. That version has mostly been replaced by something cut closer to the body, with a defined waist and a hem that stops right at hip bone rather than mid thigh, and I think that's an improvement. The shapeless kind had its uses, but it added bulk exactly where you don't want it, across the middle, and the newer cut actually does something for your proportions instead of hiding them.
On colour, the ones that get worn are almost never the loud ones. A mid grey, a soft camel, or a deep bottle green will go over more of what you already own than anything bright, because a gilet sits on top of a jumper or shirt and you need it to agree with whatever's underneath, not compete with it. Black is the obvious safe choice and it is safe, but it can look a bit flat against knitwear in a similar shade, so if you only own black jumpers, a grey or camel gilet will actually show up better than another black layer would. Quilted finishes are easier to live with day to day than the shiny technical fabrics, which read a bit more like outdoor kit than clothing.
If you've never owned one, start with a quilted style in a neutral shade, fitted rather than boxy, with a collar that sits up a little at the back of the neck. That collar detail matters more than it sounds like it should, it's the difference between looking dressed and looking like you've grabbed the nearest thing by the door. For fit, check three things with your hands before anything else: can you do the zip up fully without the front pulling, is there room enough at the shoulder seam to layer a jumper underneath without it straining, and does the hem sit at the waist or just below it rather than cutting across the widest part of your hip, which is the one spot that never looks good interrupted by a seam. Try it over the thickest jumper you actually own, not a t-shirt, because that's the test that catches the ones cut too close.
Where it earns its spot is in the layering, over a shirt with the sleeves showing, under a coat on a day that's not quite cold enough for the coat to stay done up, or straight over a jumper with jeans when you want something with a bit more shape than a cardigan but less weight than a jacket. A camel gilet, fitted at the waist, is the one I'd point anyone towards first.