A seat cushion is not where you save money, and I only had to sit on one flattened dining chair pad through an entire Thanksgiving to learn that. Thin foam looks fine in a listing photo and feels like a phone book by the second hour of a meal. The good ones hold their loft after you have sat on them every day for a year, and the bad ones tell on themselves within a month, going flat right where you sit and staying that way.
They do not belong anywhere damp or anywhere they will take real weather, so skip them on a porch or a three season room unless the cover is genuinely outdoor rated, not just outdoor colored. Most of the ones made for indoor dining chairs and desk chairs will mildew or fade if they live outside, and you will be fishing them out from under a bench after the first real rain wondering why you bothered. Inside, almost any color works harder for you if you stick to something that hides crumbs and the occasional coffee drip, so a mid tone, a dull clay, a deep olive, a faded denim blue, will outlast a crisp white one by years in a house with actual meals happening on top of it. I had a pale cream pair on my kitchen chairs for exactly one dinner party before a bit of red sauce ended that experiment for good.
In an ordinary room they sit well next to things that already have some texture to them, a wood chair with some grain showing, a linen tablecloth, a rug with a pattern busy enough to forgive a stray crumb. They do not need to match your curtains or your throw pillows, and honestly they look better when they do not try to. If your dining chairs or your desk chair are a little stiff and a little too matched already, a cushion in a different but calm tone is what keeps the room from looking like a showroom nobody actually lives in.
If you are starting with nothing, get one for whatever chair you use the most, the one at the head of the table or the desk chair you are in for eight hours a day, and get it in a cover you can take off and wash, not one you have to spot clean with a prayer. Ties that hold it to the chair matter more than they sound like they would, because a cushion that slides every time you sit down gets abandoned in a drawer within a month, no matter how good the foam inside it is.