My advice is to start thinking of metal as a color.
This is tricky, because you might be tempted to think of metals in terms of metallic colors. Fight the urge! Instead, go basic and try to look past the sheen. If I reference a couple color theory tools - value and saturation scales, to be specific - I can then match up the general tones of the metals I'm using with familiar colors.
The neutrals - white, black, and grays - aren't "colors" because they don't have a hue. Using silver, nickel, or gunmetal in a jewelry design can help lighten or ground a piece, but won't add or conflict with whatever color palette you've put together because they don't have any color properties. If I had to classify these metals as warm or cool, they'd generally be on the cool side of things (Did you know metals can be warm or cool, too, just like beads? It's true).
Gold, brass, copper, and bronze, though, all match to a shade or hue you can find on the color wheel, usually a shade of orange or yellow (which would place these metals on the warm side of the color spectrum). Because of this, using one of these metals in a design will add a new color to the mix, which may or may not work with the other colors you've chosen. If you've ever tried to work with gold or copper before and couldn't get the design to look the way you wanted it to, this could be why. There's another color at work here that you may not have accounted for.
So, if you're starting a design with a particular set of beads you want to work with, the metal findings you choose can be part of the overall color scheme and blend in, or contrast against the beads to make everything pop. How much contrast do you want between the metals and the beads?
If I use gold, there will be more contrast, because the bright yellow of gold has a different saturation and value than the pale pink. If I go with Vintaj natural brass, gunmetal, oxidized silver, or bronze, the pink will really pop and become more obvious, because the metal is so much darker. See what's happening here?
That's part of why some finished jewelry pieces pop where others don't - there's a dialog going on between the color of the beads and the metal. And if I want to take it even further, I can look at where that metal color fits into a color relationship with the other design colors. Does it fit well or is competing with a harmonious scheme?
Generally speaking, if you want minimal contrast in a design, use light beads with lighter toned metals, dark beads with darker metals, or beads and metals that are the same general hue (like yellow beads with gold metal). If you want more contrast, use opposites - dark beads with light metals, and vice versa.
Ultimately, though, I don't think there's really, truly any right or wrong decision here about the choice of metals you use. For me, that's part of a design choice; choose what will get your vision across. But if you're struggling with pairing metals with specific beads, ask yourself a few questions: What color is the metals I'm thinking of? Does it work with the color palette I have? What kind of contrast do I want?
If it feels like a lot to think about, just know that the more you do it, the more instinctual it will be. Chances are, you already know what you like or don't, as far as colors go. So, if you're struggling with certain metals, take a minute to figure out what colors those metals translate to - are they colors you like or not?
It's not necessary to match up your metals to swatches on a value or saturation scale every single time you design. So long as you have a general idea of what the color is of the metal you're working with, the effects of it in a color scheme, and a little bit of understanding about color theory, you'll be able to make intentional design choices. And you can apply this to any finding or component, really, like string or cords.
So, if you've been struggling with using different colored metals in your work, I hope that this post helps give you a new way to look at the metals you use. If you have any specific questions, feel free to leave a comment and I'll do my best to answer them!
And if you're looking for more color theory, I've got a three part introduction on my blog, if you'd like to dig deeper, with more to come: Part 1: The Color Wheel + Color Relationships, Part 2: Value + Saturation, and Part 3: Color Interactions.
Many thanks goes to the following ABS contributors for lending me use of their photos for this post! Heather Powers, Gaea Cannaday, Erin Prais-Hintz, and Kerry Bogert.