Billie Eilish talks about her synesthesia readily and specifically. In interviews she has described every song having a color, a texture, a shape, and sometimes a smell, and has walked outlets through examples from her own catalog, discussing how tracks like Bury a Friend live in particular color worlds. Her brother and collaborator Finneas has discussed the same trait, and the two have described using it as a shared production language.
Her accounts cover days of the week, numbers, and names as well as music, which suggests multiple forms alongside chromesthesia. Visual output around her records, cover art, videos, staging, has repeatedly been described by her as matching the songs’ actual colors.
As with all self-report the evidence is testimony rather than testing, but hers is unusually detailed, repeated, and consistent across years of interviews.