A good white sandal looks like leather or a clean painted finish, sits still on the foot and still looks crisp after a day of walking. A poor one goes grey at the edges within a week, the white turning the colour of pavement dust, and the straps go soft and grubby in a way that no amount of wiping ever quite fixes. You can spot the difference from right across a room, and more to the point, so can everyone else, because white shows every mark a darker sandal would hide without complaint.
White is not the right choice for a festival field, a gravel garden path, or any day involving public transport in the rain, because the splashback from a bus going through a puddle will write itself all over the front in a way you will notice for the rest of the day. They are also a poor match for anything that bleeds dye onto skin or fabric when it gets warm, so be wary of new dark denim or a cheap navy dress worn without tights, since a hot foot and a dark hem together can leave a tide mark you did not sign up for. Keep them away from very orange fake tan until it has fully set, and away from anything with a loud, busy print underfoot, because a white sandal wants a bit of calm around it to actually read as white rather than as a smudge.
Before buying, check the sole edge and the stitching, since that's where grime collects first and where a cheap sandal starts looking tired inside a fortnight. Run a finger over the inside of the strap too, a seam sitting against the side of your foot is the sort of thing that feels fine in a shop and furious after four hours on your feet. If it's a slip on, check there's an actual footbed shape to it rather than a flat slab of foam, and if it buckles, make sure the strap has another hole or two left for warmer feet by mid afternoon, because feet do swell and a sandal with no give at the end of the day is a miserable thing.
Where they genuinely work is against colour, a pale sandal under a bright dress, a block colour skirt, washed out denim shorts, anything where you want the eye going up rather than getting caught on the shoes. They sit happily with tan skin and pale skin alike, which is more than you can say for a lot of neutral shoe colours that claim to be universal and aren't.
A white sandal is a small, ongoing commitment to actually cleaning your shoes, and worth it anyway.