Lauren showed off her new hair colour at Wednesday's CoverGirl 50th birthday party in West Hollywood.
According to Lauren's hair professional, Kristin Ess, the 24-year-old had been thinking about trying out a darker hair colour for a while.
Ess told UsMagazine.com: 'It was just time to go for it.
'We've done so many variations of blonde in the past and it's fun to do something new.'
Conrad and Kristin looked through red carpet photographs for inspiration and found they loved Jessica Biel and Mila Kunis's 'Brazilian brown' shade.