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 colourist to deposit a darker colour on your roots — preferably one that is two or more shades darker than your natural hair," said Schaudt. "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 colour being applied thru the mids and ends."
The secondary colour should be the brighter of the two shades. "If your ends are pre-lightened, you'll paint from the tips up. This colour should marry with the base colour, giving a noticeable meeting point."
Just note: "If your hair is not already pre-lightened, you'll need to first marry the lightener with the base colour."