The general rule is one or two colors, plus a metallic accent of gold or silver. You don't want to look like you acquired your taste at Clown College. The gold or silver accents are usually added because they reflect light for theatrical drama on stage. Gold is warm, and silver is cooler. Because silver is colorless, it reflects whatever light shines upon it, like a mirror.
Copper is a third, more subdued, earthy metallic.
Or the metallic accent could be the whole bra or whole belt. For example, a gold-coin covered bra and belt with a blue skirt or harem pants. Wearing solid gold or solid silver all over is the most flashy, theatrical statement you can make, so if you do this, be confident that you can carry it off without looking like your costume is wearing you. Using a single predominant color for belt, bra and skirt creates a unified effect that generally makes you look taller, so this is a good choice for most dancers. For instance, a red skirt, belt and bra with red and gold fringe and gold jewelry.