What a setting does to the colour of a stone

A sapphire is transparent, which means it does not only transmit light, it also picks up what surrounds it. The metal of the setting is the nearest and largest of those surroundings, and it is never colour-neutral.

Yellow gold reflects warm light back into a stone. Against a blue sapphire that can slightly mute the colour or push it toward green; against a yellow or padparadscha sapphire it can reinforce and enrich it. White metals, platinum and white gold, keep the reflected light cool and let a blue read as clean as it is.

This is why a yellow shank with a white setting head is such a durable combination rather than a compromise. The warm metal sits against the skin where it flatters the hand, and the white metal does the work where it touches the stone.

A bezel amplifies the effect, because it wraps metal all the way around the girdle. Prongs touch far less of the stone and let more outside light in from the sides. None of this is a rule about what to choose. It is a reason to see the stone and the metal together before the piece is made.