2018 Lollapalooza Billie Like we said, lime green — otherwise known as slime green — is Billie's colour. At 2018's Lollapalooza, she rocked a bright t-shirt with her name on it and matching beanie. What to wear: A green t-shirt and matching beanie with a pair of men's swim trunks as shorts. Image Source: Getty / Tim Mosenfelder