My first pair of slippers as an adult were the kind you get free with a magazine, thin as a biscuit, and they taught me that sole thickness is not a detail you can skip past. Three weeks on a tiled kitchen floor and my heels were cold through the whole winter.
If you've got none in the house right now, start with a simple slipper boot or a mule with a proper rubber sole, not the slipper that's basically a sock with ambitions. Pick them up before you buy and bend the sole in your hands. If it folds flat like paper, it'll feel like nothing under your feet by February. You want something with a bit of resistance, a sole that springs back, because that's what's between you and a stone floor at six in the morning when you're not thinking about fashion, you're thinking about the kettle.
On colour, I've gone through every phase of this. Bought a cream pair once, wore them twice, they looked like they'd been dragged behind a car by the third week. Grey, a deep burgundy, a proper tan suede, anything that isn't going to show the inevitable tea splash or the mark where the dog's been sat on your foot, that's the territory worth staying in. Pale colours are lovely in the shop and a liability in your actual house. A plain, slightly dark shade also just goes with more, which matters more than you'd expect for something nobody else is meant to be looking at.
They pair with everything, honestly, which is the whole point of a slipper, but the combination that works hardest for me is leggings and a long jumper, because the proportions do something flattering without trying, and the slipper boot disappears into the hem rather than cutting your leg off at an odd point. Same with a dressing gown over pyjamas, the slipper needs to be low profile enough that it doesn't bunch the hem up when you're walking, which sounds like a tiny thing until you've tripped on your own dressing gown at the top of the stairs.
Try them on with the socks you'd actually wear at home, not bare feet in a warm shop, or you'll buy a size that pinches the second winter starts properly.