What does the term "foil" refer to in hair coloring?

The term "foil" in hair coloring refers to sheets used to separate sections of hair during the coloring process. This technique is common in highlighting, where sections of hair are wrapped in foil to isolate them. The foil serves multiple purposes: it helps to keep the color applied to the sections from bleeding onto adjacent hair, it enhances the effectiveness of the color process by retaining heat, and it allows for more precise application.

Using foils permits a stylist to control the amount of lightening or darkening that occurs on specific strands, resulting in a more controlled and defined outcome. This method is particularly effective for achieving multi-dimensional color effects and can create various highlighting techniques, including balayage and babylights.

The other options pertain to different aspects of hair coloring but do not accurately describe what "foil" specifically refers to in this context.