Green rugs photograph beautifully and then arrive looking nothing like the swatch, that's the warning. Green covers such a wide range, from a gray sage that reads almost neutral to a bottle green so saturated it behaves like a dark navy, that buying from a thumbnail is a genuine gamble. I'd get a sample swatch or at least check it under your own lights before committing, because the emerald that looked rich online can turn muddy and a bit tired under a warm bulb at seven in the evening, which is exactly when you're lying on the floor with a glass of wine regretting things.
Where it goes wrong is usually other greens, which sounds obvious until you're standing in a room with a green sofa, green cushions and now a green rug, and it all starts to look like you're trying to recreate a golf course indoors. It also fights with busy wood tones, a heavily grained orange oak floor under a deep forest rug can look like two different decades arguing with each other. Rooms with a lot of plants need a lighter hand too, since a jungle colored rug under actual plants just doubles down rather than balancing anything out. If your walls are already a warm terracotta or a strong yellow, a cool green rug can sit there looking like it wandered in from another house. It's the wrong choice entirely if you're after something that blends into the background, because even a muted sage has enough presence to be noticed, it's not a rug you can forget about once it's down.
Sizing is where most people actually lose money, not on the color. Standard advice is front legs of your furniture on the rug, which I've broken myself in a rented apartment because I measured the room and not the actual seating arrangement, ending up with a rug that floated in the middle like a green life raft nobody's sofa was touching. Measure the space between furniture pieces, not just the walls, and leave a border of floor showing, somewhere between eight and eighteen inches depending on the room, or it starts looking like wall to wall carpet that shrank in the wash. Underfoot, a denser green wool rug has real weight to it, it sits flat without curling at the corners and sounds solid when something heavy gets set down on it, where a thin flatweave in the same color will slide around and feel more like a placeholder than a finished room.
Would I still like this color if the room's lights were off and I was just feeling it with bare feet?