The Nanny McPhee star was shocked when her 10-year-old daughter Gaia urged her to dye her locks darker because she shouldn't have fair hair when she's 50.
But Thomson is adamant she won't heed the youngster's advice until she turns 60, when she will opt for a more mature look.
The actress reveals, "She says, 'It's got to go'. I say, 'Look Gaia, I am only 50, I will get rid of it when I am a bit older' and she says, 'You are too old for blonde hair, Mum.' So when I am about 60 I will just shave my hair off and go salt and pepper. You can't have blonde hair at the age of 60."