A 14kt multitone piece works across more skin tones than single-metal jewelry because the combination of warm yellow and cool white gold creates a neutral overall effect that neither color alone produces.
Visually Speaking
London blue topaz sits at the darkest end of the blue topaz spectrum: deeper and cooler than Swiss blue's vivid medium, and significantly more saturated than sky blue's pale brightness. The color differences between grades is the direct result of variations in the irradiation and heat treatment process applied to the same colorless starting material.
Elemental Detail
Hold an 18kt multitone gold piece next to a 14kt one and you'll notice the color difference immediately. The higher gold content in 18kt alloys produces deeper coloring across each zone, in turn allowing for the contrast between those zones to be very visually pronounced.