I always thought of polo shirts as something for golf courses or school uniforms, nothing to do with me, until I borrowed one off a friend for a car boot sale because everything else I owned was in the wash. Comfortable, didn't need ironing, and I didn't spend the whole day pulling at a neckline. That was the end of my snobbery about them.
Colour wise I steer towards navy, a soft stone or a proper bottle green rather than anything too crisp or too pale. White looks brilliant on the hanger and shows every bit of tea, foundation and whatever your day throws at it within the hour. A knitted polo in a heathered grey or a muted rust is the easiest thing to wear without thinking about it, and it hides a multitum of creases that a cotton pique one won't forgive.
These sit well with jeans, obviously, but the combination I reach for more in autumn is a knitted polo under a collarless coat, with the collar just peeking out at the neck. In summer I wear one with a cotton skirt and flat sandals and it looks like I've made an effort without having made any effort at all. They're also handy layered under a sleeveless jumper on days that can't decide what season they're in, which round here is most of them.
Before you buy, check the collar holds its shape rather than rolling flat after one wash, that's the thing that tells you whether it'll still look smart in a month or start looking like a crumpled bit of old school uniform. Have a feel of the placket too, the bit with the buttons, because on cheaper ones it can pull and gape across the chest no matter what size you've gone for. With knitted styles specifically, stretch the hem gently in the shop. Some spring back fine, some give up and go wavy by the second wear, and you want to know that before you've handed over your money, not after.
They struggle a bit in the height of summer if the fabric's too thick, and in proper cold they're more of a layering piece than something that'll do a job on their own, so don't expect one to replace a jumper when it's genuinely freezing. Spring and early autumn are really where they come into their own, worn alone on a mild day or doubled up when it turns.
Next time you're looking at one, check the collar first, how it sits and whether it keeps its shape when you fold it back, because everything else about how smart the shirt looks depends on that.