Neutral frames get called boring more than almost anything else in a house, and I get why, but I land on the other side of it. Boring is the point. I went through a phase of mixing gold, black, and a weird rose colored set I found on clearance, and the wall above my couch ended up looking like three different people had decorated it, because in a way they had. Neutral frames, the white, natural wood, and warm wood tones, let the photos do the talking instead of fighting the frame for attention.
Where they struggle is against anything already busy. A heavily patterned wallpaper, a gallery of mixed metals, a room that's already got a lot of texture going on with rugs and throws, that's where even a plain frame starts to feel like clutter rather than calm. They also look a little lost against very dark walls unless you go with a deep wood tone that can hold its own. As for sizing, the mistake almost everyone makes, myself included for years, is buying frames to match the wall instead of the photos. A huge expanse of wall doesn't need one huge frame, it usually needs several modest ones grouped with real intention, two to three inches apart, hung so the center of the group sits at roughly eye level, not at the dead center of the wall like it's a piece of art in a museum. Too many people hang the whole group a foot higher than it should be because they're thinking about the furniture below it instead of the person standing in front of it.
If you're starting from nothing, go with a simple wood frame in a mid tone, something between honey and walnut, because it reads as neutral without going cold the way bright white can in a room with warm light. A thin molding over a thick one, too, since thick frames can swallow smaller photos and make the whole thing feel heavier than it needs to be. From there, neutral frames sit easily with almost anything ordinary, a linen covered couch, a wood dining table, a bookshelf that's already a little overstuffed. They don't ask the room to change around them, which is rarer than it sounds.
What they won't do is make a bland room interesting on their own. A neutral frame holding a great photo looks wonderful. A neutral frame holding nothing in particular just looks like an empty frame that happens to match the trim.