A rubber trainer should look like it doesn't care what you think, and that's exactly why it works. The cheap ones try too hard to look like trainers, all stitching and fake texture, and within a few wears the white bits go grey and the rubber starts to crack along the fold where your foot bends. A proper one is moulded in one piece, nothing to peel or split, and it ages by going slightly matte and softer underfoot rather than falling apart.
If you've none of these, start with a low, plain one in a single colour, nothing split tone, nothing with a chunky contrast sole, because that's the version that goes with the most things without you having to think about it twice. Wipe them down with a damp cloth and they come back looking new, which sounds obvious but the first pair I bought I tried washing on a hot cycle because I couldn't be bothered wiping them, and they came out with one shoe a half size bigger than the other, properly warped, like something from a cartoon. Buy once, wipe forever, that's the whole maintenance plan. They're brilliant in the rain, which is reason enough to own a pair if you live anywhere near weather, and they don't go soft and sad the way a canvas trainer does after one proper downpour. They pair with the ordinary stuff, jeans rolled at the ankle, a straight leg trouser, a plain dress on a day you want to walk further than your nice shoes would allow. They look a bit wrong with anything too dressed, a slippy satin skirt or anything with real tailoring, there's a point where the trainer starts fighting the outfit instead of sitting under it quietly, and you'll feel it the second you catch yourself in a shop window.
Colour wise, go for something that reads as a neutral even if it isn't quite one, a soft putty, a proper white, a black that's actually black and not a greenish fake black that shows up under certain lights. I'm not going to pretend I reach for beige by choice, I don't, but I'll admit a stone coloured rubber trainer has never once let me down the way a loud colourway has, because the loud ones date fast and the stone ones just keep going, season after season, like they've nothing to prove.
Whatever you're already wearing on a day you can't be bothered with proper shoes, this is the thing that makes it look like a decision rather than a retreat.