The first take on the trend is the opposite of what you see on Billie Eilish: drenched ends. "If you want to go darker on top and funkier on the bottom, ask your colorist to deposit a darker color on your roots — preferably one that is two or more shades darker than your natural hair," Schaudt said. "This creates depth within the base and complete saturation of the hairline and part. By first adding a deeper pigment you're creating a perfect backdrop for the brighter color being applied through the mids and ends."
The secondary color should be the brighter of the two shades. "If your ends are prelightened, you'll paint from the tips up. This color should marry with the base color, giving a noticeable meeting point."
Just note: "If your hair is not already prelightened, you'll need to first marry the lightener with the base color."