The one thing to know before you part with any money is that a walking boot is only as good as the socks you put it on with, and the fit has to be done at the end of the day when your feet have swollen a bit, not first thing in the morning when they're at their smallest. Buy them small and you'll know about it three miles into a hill with no way back except forward. That's the whole lesson, really, everything else is detail.
They don't sit well with skinny jeans tucked in tight, the bulk at the ankle fights the line and you end up looking like you're wearing someone else's trousers over someone else's boots. Looser straight leg or a cropped trouser that stops just above the cuff gives the boot room to do its job and looks like you dressed on purpose rather than in a hurry. For the fit itself, go up half a size from your usual and wear the actual socks you'll walk in when you try them, thick ones, because a boot that's fine with a thin cotton sock will crush your toes the first time you put proper wool on. Walk a slope in the shop if there's one to find, even a ramp, because that's where you learn whether your heel lifts out the back, which is the fit problem that ruins more pairs than anything else. A heel that lifts means blisters by lunchtime, no matter how lovely the boot looked on the shelf. Check the width across the widest part of your foot too, not just the length, because a lot of boots are built narrow through the middle and your foot will tell you about it loudly after an hour.
On colour, I'll say the thing I always say and then immediately break my own rule, because a boot in a soft moss or a dark heathered grey goes with nearly everything you own and never looks like it's trying too hard, which matters when it's getting muddy every weekend anyway. But I've a pair in a deep rust brown that I didn't expect to love and now reach for over the grey ones more often than not, because it sits nicer against green and against navy than plain tan does, and it hides the inevitable scuffing far better than black does. Black looks sharp on day one and tired by day twenty, every scrape shows. Go for something with a bit of warmth in it and you'll get more wear out of looking at them, never mind walking in them.
Best for anyone who actually walks regularly rather than owning boots as an idea of walking, because a boot that's never tested on a slope will fail you exactly when you need it not to.