The thing people get wrong with flat sandals is assuming flat means fine, no thinking required, stick your foot in and off you go. Then three hours into a day of pavements and standing about, the straps have rubbed a line across the top of the foot and the sole's gone paper thin under the ball of the foot, and you remember why last summer's pair got binned in September.
Sizing on these is sneakier than it looks because feet spread sideways in the heat, so a sandal that's snug in the shop can feel like a tourniquet by midafternoon. Go for a toe box with a bit of room rather than one that just skims the edges, and check the back strap sits on the ankle bone without digging, because that's the spot that blisters first. If there's any buckle or adjustable strap, use it, don't just accept the factory setting, half the fit complaints I hear are from women who never moved the strap off the hole it came on. Wide feet want a rounder toe shape and a strap that doesn't cut straight across the widest part of the foot, narrow feet want to watch for gaping at the sides when you walk.
What a flat sandal clashes with is anything too fussy below the ankle, heavy ankle socks underneath them being the obvious one, but also hems that are so long they catch under the sole and trip you on steps. Thick, clumpy straps don't sit well with delicate, floaty fabrics up top either, it ends up looking like two different outfits got dressed by two different people.
The gap between a cheap pair and a good one shows up fast. Budget sandals usually have a stiff, flat insole with no shape to it, so your foot does all the work and the sandal does none, and the straps are often a synthetic that doesn't soften, it just cracks eventually along the fold lines. A well made pair has a footbed with some contour to it, leather or a leather lined strap that gives slightly with wear instead of staying rigid, and stitching or edging that's finished properly rather than left raw and fraying within a month. Pick one up and flex the sole, if it barely bends, your feet are going to know about it by the second wear.
On colour, tan and a soft stone work with nearly everything you already own and don't show dust the way white does, while true black is the easiest of the lot for anything smarter. Bold colours are lovely for one specific outfit and then sit in the cupboard the rest of the year, so buy those with your eyes open.
Wipe leather straps down with a damp cloth after a dusty day out and let them dry away from direct heat, so they don't stiffen and crack.