There's a particular stretch, somewhere around the first dry weekend after a grim stretch of winter, when ankle boots start to feel wrong and you want a trouser that does some of the dressing up for you without you having to think too hard. That's when flared jeans come back into their own. They carry a jumper and flat shoes like nothing else does, and they make the rest of the outfit look like you planned it, even on a day you really didn't.
Before you buy, check where the flare actually starts, because that's the whole decision. A flare from the knee behaves completely differently from one that starts at the thigh, and the label rarely tells you which you're getting. Check the inseam too, properly, with the heel height you'll actually wear, because flares live or die by whether they skim the floor or stop short and look like you've grown two inches since trying them on. And have a feel of the fabric with some stretch in it. A flare with zero give across the thigh will fight you sitting down on a long car journey, and I say that as someone who once spent a four hour drive perched sideways like I was riding side saddle.
Where they're the wrong choice is anything that needs you moving fast or climbing over things. A step ladder, a toddler, a train with three minutes to make the connection, all of these are better done in a straight leg. The hem picks up dirt and rain in a way a slim jean never does, and if you're someone who's hard on the bottom of your trousers, they will show it first. They also don't love a hot wash. Mine went from a proper flare to something closer to a boot cut after one accidental 60 degree cycle, so cold water and the gentle setting, every time, if you want them to still move the way they did when you bought them.
For colour, a mid or dark wash is the easiest thing to live with by a long way, because it hides the fading along the hem that happens no matter how carefully you wash them. White or pale flares look wonderful for about three wears and then show every single thing that touches the ground. As for what they clash with, keep them away from anything else with a lot going on below the knee, a chunky boot with buckles, a big platform sole, a skirt length underneath that peeks out. The whole point of the shape is one clean line from hip to floor, and it only works if nothing else is competing for attention down there.