They drank something "special" during a festival. My sim started glowing purple when she went to the Hijinks festival and "joined the pranksters" by drinking from the purple fountain. Hope this helps!

Why is my sim glowing purple Sims 4?

The Specter Sprinkles buff makes you glow purple.

What does it mean when a sim glows pink?

Aliens especially glow when certain emotions are strong. Pink for flirty, blue one shade for focused and a deeper shade for extreme sadness.