Which term refers to the visual depth or contrast in hair color?

The term that refers to the visual depth or contrast in hair color is dimension. Dimension in hair color involves the use of different shades, tones, and highlights to create a multi-faceted look that adds depth and interest. This can be achieved through techniques such as balayage, foiling, or color melting, which incorporate varying levels of lightness and darkness to produce a three-dimensional effect.

In contrast, tone relates more to the specific hue of the color itself, describing whether it is warm, cool, or neutral. Color theory is a broader concept that encompasses the principles of how colors interact, which can inform choices in hair coloring but does not specifically address depth. Illumination typically refers to the way light affects color perception, but is not directly about creating depths within the hair color itself. Thus, dimension is the term that most accurately describes the effect of visual depth in hair coloring.