The band is the ring's wardrobe, and the choice is smaller than the showroom implies: gold in three colors, at two sensible purities, or platinum. Each asks something slightly different of its owner.
Color is a matching exercise
White metal reads modern and makes near-colorless stones look colder and cleaner. Yellow is the classic, currently enjoying one of its periodic returns to fashion, and it flatters warmer stones enough to relax a color grade. Rose is the romantic outlier that photographs beautifully and matches less of what she may already own. The audit is simple: open her jewelry box, count the colors, and vote with the majority.
Purity is a durability trade
Fourteen-karat gold carries more alloy, which makes it harder and better suited to a ring that never comes off. Eighteen-karat is richer in color and softer in temperament. Both are correct; one is more carefree.
White gold's brightness is a rhodium plating that thins over the years and wants occasional renewal. Cheap to do, easy to forget.
The platinum question
Platinum is white to the bone, noticeably denser, kind to sensitive skin, and more expensive at the register. It does not wear away; it develops a soft patina that polishing can reverse. For a daily white-metal ring owned by someone who will never remember to replate anything, it is the buy-once answer.
The short of it: match the metal she already wears, default to fourteen karat, and pay for platinum only when forever-white with zero errands is the point.