Define "brassiness" in hair color.

Brassiness refers specifically to unwanted warm tones that appear in color-treated hair, typically manifesting as shades of yellow or orange. This phenomenon often occurs when lighter hair, such as blonde or highlighted hair, begins to show warmth due to chemical reactions, fading, or the natural undertones of the hair. These warm tones can be particularly undesirable for those seeking a cooler, more neutral hair color, which is why many stylists recommend using specific shampoos or toners to counteract brassiness and maintain the desired hue.