Most people buy chair cushions to match the chair, then wonder why the thing slides off the second someone leans forward to reach for the salt.
The limits are worth knowing before you spend anything. A cushion is not going to rescue a genuinely uncomfortable chair, the kind with a seat pan angled wrong or slats that dig in no matter what you put on top. It softens, it doesn't fix structure. And ties matter more than people expect. A cushion with no ties, or flimsy ones stapled on rather than sewn through, will walk itself sideways within a week, especially on a chair that gets pulled in and out from a table. If you have dining chairs that already fit well and just need a bit of comfort and some color, a well made cushion does real work. It sits well in a kitchen with a wood table and mismatched chairs, in a reading corner with one good armchair that needed softening, on bench seating where a run of cushions ties a whole row together without you needing to reupholster anything. It also sits well as a quick fix for chairs that don't quite match each other, since a shared cushion fabric pulls them into one set even when the frames are different.
Where it goes wrong is usually about mismatch rather than the cushion itself. A thick, overstuffed cushion on a delicate chair, something with a slim cane seat or a slight antique frame, looks like a pillow fight that wandered into the wrong room. And a busy pattern on a cushion sitting on top of an already busy chair, carved wood, upholstered back, whatever, turns into visual noise nobody asked for. The difference between a cushion worth keeping and one you'll be replacing by spring comes down to a few things you can actually check with your hands. Good foam holds its shape and springs back when you press it, cheap foam flattens into a pancake within a month and stays that way. Ties should be sewn into a seam, not glued or stapled on as an afterthought. And the cover should have enough fabric weight to resist pilling where the chair back rubs against it, which is the first place a cheap cover gives out. Piping along the edge, if it has it, should be tight and even, not wavy.
If I were starting from nothing, I'd pick a solid in a mid tone, something like a clay or a muted ochre, before I went anywhere near a pattern.