Carrie Bickmore's grandmother had a brutally honest reaction to her brown hair.

There’s nothing nicer than those first weeks with a brand new hairstyle, but sometimes the most… interesting reactions come from those closest to us.

The Project co-host Carrie Bickmore revealed it was her grandmother who gave the most unexpected reaction to the new do.

“Oh, Nan doesn’t like my hair,” Bickmore told her co-hosts Waleed Aly, Peter Helliar and Tom Ballard on tonight’s episode of the Channel 10 panel show.

The 35-year-old went on to share how her grandmother was so surprised by the change from blonde to brunette, she thought her granddaughter had been replaced.

“Nan watched the TV for two weeks and thought I’d gone on holiday,” she said.

“And they’d found a girl with… different coloured hair that sounded like me to fill in for me.”

Bickmore revealed she traded in her golden hair for a chocolate brown shade in late August this year. 

At the time, the co-host said the change was simply the result of a desire to try something new.

“I’ve had the same colour hair for 35 years and I was bored and I thought, ‘Why not?'” she told viewers.

“I’m giving it a whirl!”

Bickmore also shared how her son Oliver was similarly confused about the change.

"My son's not dealing with it well... quite yet," she said.

"He looks at me and goes, 'When's old Mummy coming back?'"

Proof you can always rely on kids and grandmas to offer an honest opinion.

Listen to Mia Freedman's interview with Carrie on the No Filter podcast: