The thing nobody mentions about footrests is that the height matters more than the look, and you won't know if you got it wrong until you've sat with it for a few weeks. Too low and you're still hunching to compensate. Too high and your knees end up somewhere near your chest, which is its own kind of uncomfortable. The good ones get the proportion right relative to an average chair seat, and that's not something you can judge from a photo.
Construction is where the cheap ones give themselves away first. A footrest takes a strange kind of pressure, feet pressing down and shifting around, sometimes a full body weight when someone decides to stretch out, so the legs and the joints where they meet the frame need to be solid, not just glued dowels that start to wobble after a season. Look for actual joinery or a solid base if it's upholstered, and check that the padding has some density to it rather than a thin foam that goes flat within a year. On a wood one, solid legs with a proper finish will outlast a stained veneer by years, and you'll feel the difference the first time you rest a heel on the edge.
Color and finish decisions here are lower stakes than most furniture, which is part of the appeal. A neutral upholstered top in a weave that hides use, something like a soft gray or a warm oatmeal, blends with almost anything and doesn't show the wear that a pale linen will. If you're going with wood, a medium or dark tone hides scuffs better than a pale, raw finish that marks the second a shoe grazes it. Leather or a faux leather in a brown or black tone ages well and actually looks better with a little character over time, unlike fabric that just looks tired.
These earn their keep most in living rooms next to a favorite chair, in a reading corner, or tucked under a desk in a home office where you're sitting for hours at a stretch. They're especially good in smaller homes where a full chaise or recliner isn't realistic, since a footrest gives you the stretched out feeling without eating the square footage. Where they don't make sense is in a high traffic path or a room with little kids running through, since they become a tripping hazard fast, and they're also not much use if your seating is already low to the ground, where your feet end up at an odd angle instead of a restful one.
Whatever chair you already love, this is what finally makes it feel like somewhere you'd actually stay for a whole evening.