Karat tells you how much of the metal is pure gold. 14K is 58.3 percent gold, and the other metals in the mix make it harder and more scratch resistant, which suits a chain or pendant you wear constantly. 18K is 75 percent gold, with a deeper, butter-rich color and a softer feel, so it rewards a little more care.
Neither is a mistake. A necklace takes fewer knocks than a ring or a bracelet, so many people happily wear 18K at the neck. The two colors also differ side by side: 14K reads as a bright, classic yellow, while 18K looks warmer and closer to the color of pure gold. If you already own gold pieces you love, match their karat so the tones agree.
Warm metal hides a warmer diamond well, which is good news for your budget. Against white metal, a hint of warmth in a stone shows; set in yellow, that tint is masked by the metal around it. A near colorless stone in the G to J range looks bright in a yellow gold diamond necklace, and spending extra for the colorless D to F grades buys a difference few people will see.
Color shows more in larger stones, so that advice matters most on a bigger pendant. On a pavé line of tiny stones, it hardly matters at all, and you can put the money toward cut, which drives sparkle.
Know which one you are buying. A solid piece is gold alloy all the way through, so scratches can be polished out without losing the color. Gold vermeil necklaces are sterling silver coated with a layer of gold of at least 10K, thicker than ordinary plating but still a coating that can wear with heavy use.
Vermeil is a fine way to wear a trend for a season. For a piece you mean to keep for years, or one you plan to hand down, the real thing is the better buy.
Wearing yellow next to white gold or silver is no longer a rule to avoid; it looks deliberate when you repeat each metal at least twice, for example a yellow chain and yellow earrings with a white gold watch. Matching yellow gold earrings tie a look together, and the wider yellow gold jewelry range makes it easy to build a set in one tone.
Rose gold sits comfortably beside yellow too, since both are warm; it gets its pink color from copper, so the two read as cousins rather than opposites.