Busy Philipps dyed Michelle Williams' hair pink because she was "curious" about the makeover.
The 38-year-old actress took to social media to document the pampering session with the former 'Dawson's Creek' star on her Instagram Story last week, which saw her colour Michelle's blonde pixie crop a light rose hue whilst she sipped on a glass of wine.
But the 'White Chicks' star has revealed she had discussed changing the 37-year-old's hair colour in advance.
She told Us Weekly: "We talked about it."
And Busy thinks Michelle - who has 11-year-old daughter Matilda with her late husband Heath Ledger - went ahead with the new hair style because she has "always" thought about changing her appearance.
Busy explained: "I think Michelle had always been curious about dyeing her hair."
Busy also boasts pink locks, and she wasn't afraid to experiment with her hair colour because she doesn't think changing her hair to pink is "much of a commitment".
She said: "The pink washes out so fast, so it's not that much of a commitment.
"I think that's why so many people are dyeing their hair now.
"It's just a fun thing to do and it's not going to ruin your hair or last forever."
The 'Manchester By The Sea' star was meant to leave the dye on her tresses for 20 minutes, but her and Busy decided to cut down the suggested time advised so her hair to opt for a more subtle change.
Speaking at the time of the beauty session, Busy said: "You have to put it in for 20 minutes. I have it in my hand. It's like pink slime. We will do it for like five minutes."
However, Michelle wanted to slash the time the dye was meant to stay on her hair even more.
Michelle replied: "Three minutes, two minutes, honestly like after one minute we should stop."
Busy compromised with Michelle and after massaging the product into "the little pixie's hair", they washed out the substance in "two minutes".
As Busy slurped on a glass of wine, she said: "Guys, I am putting the pink in. I am massaging the pink into the little pixie's hair. Michelle's hair is going to be perfectly millennial pink. She is going to wash it out in two minutes and it is going to be amazing."