White gold or platinum are the traditional metals for halo engagement rings. However, as you consider your halo options, you may discover that yellow gold is the ideal compliment. Yellow gold is an excellent choice for diamonds with a yellow tint since it minimises the tint (white gold would enhance it, which you probably don't want unless you have a real yellow diamond). Two-tone settings might be a fun option. Rose gold is also becoming more popular as an engagement ring metal. It has an antique air to it, and its soft, pinkish colour may complement a variety of coloured gemstones, such as a green peridot.