A green dress earns its spot for the days when black feels like giving up and you still want something you don't have to think hard about pairing. It works as the one bright, decided thing in an outfit, so the rest of your choices get easier, not harder. Shoes, bag, coat, all sort themselves out once the dress has made the main decision for you.
Before you buy, check the shade in daylight and not just under shop lighting, because greens shift more than almost any other colour depending on the bulb above the rail. Hold it near your face and look at your skin rather than the dress itself, since that tells you more than any description on a label. Check the lining if there is one, because a lot of mid range green dresses are made from fabric that's thin enough to need it, and a dress without a lining in a lightweight green can go translucent in exactly the moment you don't want it to. Look at the seams around the waist and underarm too, pull them gently, and if they pucker or strain already on the hanger they'll only get worse on you.
Fit goes wrong on green dresses in the same two places it goes wrong everywhere else, the bust and the hips, but the colour makes it more obvious if you get it wrong, since a dress that's tight across the stomach in a strong green draws the eye exactly where you don't want it. If you're between sizes, go up rather than down, you can always take a dress in at the sides but you can't add fabric back. Sit down in the changing room before you decide, not just stand and turn, because a lot of dresses that look perfect standing still ride up or gape once you bend. As for what it clashes with, be careful layering it under a coat or cardigan in red or orange, the two colours fight for attention rather than working together, and a lot of browns can make a green dress look murky rather than rich. Navy, cream, grey and black all sit well against it instead.
The difference between a green dress that lasts and one that disappoints usually comes down to the weight of the fabric and how the colour was applied. Cheaper dyes fade unevenly after a few washes, often lighter at the shoulders and hem first, so check the care label and avoid anything that insists on hand washing only unless you know you'll actually do it. Good buttons and zips feel solid when you work them, not flimsy or stiff, and a hem that's been properly finished won't fray within a season. If you're choosing a shade to start with, go for a deep bottle green, it's forgiving, it photographs well in ordinary light, and it doesn't date the way brighter greens can.