Rhodium does. White gold is gold mixed with paler metals, then plated with rhodium, a hard silvery metal that gives the gleaming finish you see in photos. Without that layer, the gold underneath has a softer, slightly warmer tone.
The plating is thin, so friction wears it away over the years, usually first where the chain rubs at the back of the neck or around the clasp. A jeweler can replate the piece and bring the full shine back. On a necklace this happens more slowly than on a ring or bracelet, since the neck sees far less contact than the hands.
White gold and platinum show warmth in a diamond more than yellow or rose gold do. A stone with a faint yellow tint can look creamy against a cool, pale setting, so it pays to choose higher on the color scale here, in the colorless D to F range or the top of near colorless.
How much this matters depends on size. Color shows more in large stones, so a single bigger pendant deserves the closest look. For a white gold diamond necklace with many tiny stones, the effect is the sparkle of the whole line, and small color differences blend away. Put the savings toward cut instead, since cut matters most for sparkle.
Start with the pendant. A four prong solitaire shows the most diamond, a bezel wraps the stone in a rim of the same cool metal for a smooth, secure look, and a halo adds a ring of small diamonds around the center so the pendant reads bigger.
Then set the length. Around the collarbone means 16 inches; just below it, the most worn spot for a pendant, means 18; and 20 inches falls a little lower to clear a higher neckline.
All three look white, but they age differently. Platinum is naturally white, so it never needs plating; it is denser and heavier, hypoallergenic for most people, and develops a soft patina rather than losing metal when scratched. Sterling silver costs less and is pale too, but it tarnishes over time and needs polishing; compare sterling silver necklaces if a lighter budget matters most.
White gold sits between them: lighter than platinum, harder wearing than silver, with that plated shine. For pieces in a single karat, see 14k white gold styles, or match earrings and bracelets across the white gold jewelry range.