Which of the following is NOT a method of hair color application?

Smoothing is not a method of hair color application because it primarily refers to techniques aimed at reducing frizz and enhancing the sleekness of the hair. This may involve using specific products or tools like flat irons, rather than applying color to the hair. In contrast, techniques like the foil method, ombre, and balayage are all specific hair coloring applications aimed at altering the hair's color either through highlighting, creating a gradient, or painting the color directly onto the strands for a sun-kissed effect. These techniques are integral to the artistry of coloring hair, while smoothing focuses instead on the hair's texture and manageability.