Which rule is used to determine the level of hair color for clients with gray hair?

The gray card rule is particularly useful in determining the level of hair color for clients with gray hair because it provides a standard way to assess the percentage of gray present in a client’s hair. This rule helps colorists evaluate how much of the underlying pigment will affect the final color result during the coloring process.

When a client has gray hair, it is essential to understand that gray hair can reflect color differently compared to pigmented hair. The gray card serves as a reference that allows the colorist to see how much gray hair is present and what level of coverage or color may be necessary to achieve the desired effect.

In contrast, the other options, while related to color theory and hair coloring techniques, do not specifically address the unique considerations for clients with gray hair. The color wheel rule provides guidance on color relationships but doesn't directly apply to measuring gray levels. A shade guide typically shows various colors and shades available but lacks the specificity needed to gauge gray coverage. The lightening guide focuses on how different hair types respond to lightening processes, not specifically dealing with gray hair classification or the assessment of gray presence.