Does the set actually need to match, or is that just something we tell ourselves to justify buying two items instead of one? Matching helps the proportions sit right under clothes, especially if the brief or the bra band is cut to the same stretch, but the real reason to buy a set rather than mixing and matching is fit consistency, not the look of it in a drawer nobody else opens.
Getting the fit right starts with the bra, because the knickers will almost always behave if the top half does. Band size matters more than cup size, the band should sit level all the way round and not ride up at the back by the end of an evening, and the gore, the bit between the cups, needs to lie flat against your sternum rather than bridging over it like a little tent. If the wire is poking you under the arm or digging into the side of your bust, that's not something that beds in with wear, that's the wrong size, full stop. Briefs are simpler but get judged just as harshly, the elastic at the leg should sit without a red line appearing the second you sit down, and anything that needs constant tugging back into place by lunchtime was never right to begin with.
The difference between a set that lasts and one that's in the bin by fourteen washes is almost always in the bits you don't look at first. Cheap lace frays at the scalloped edge within a month. Good elastic keeps its stretch after a hundred washes, bad elastic goes slack and the whole thing starts looking tired even though technically it still fits. Hooks and eyes matter more than people expect, three rows rather than two gives you room as your body changes slightly through the month, and a metal slider on the strap that doesn't dig into your shoulder is worth noticing before you buy, not after. Seams should be flat against the skin, not sitting proud and visible through a t-shirt, which is the detail that gives away a bargain set every time, however pretty the lace looks on its own.
If you've got nothing decent in the drawer at all, start with something plain, a well constructed set in a skin tone or black, nothing with padding you don't need or a plunge you'll never wear anything low enough to show off. It goes under everything, a shirt, a jumper, a dress with an awkward back, and it teaches you your actual size before you start spending on the sets with the nice detailing that only reveal their fit problems after you've already cut the tags off.
Once you know your size properly, the rest gets much easier to choose with confidence, and a decent plain set quietly makes everything else in the drawer look better by comparison.