Kate, 35, revealed that the daring shade was actually her 10-year-old son Ryder's idea.
"We just thought, you know, it's breast cancer awareness month -- it was actually Ryder's idea," she explained to ET. "Ryder was like, 'Mom, let's do something for breast cancer awareness,' and I said 'Great,' and he said, 'Let's dye our hair,' and I said, 'OK, let's do it.'"
And of course, her stylish son had even more wild plans for his own locks.
"I let him do it," she smiles. "He did more, like, pink parts and some blue, but yeah, it was cool and fun."
It turns out dying their hair pink for breast cancer awareness month was a family affair, with her mother Goldie Hawn, 68, also getting in on the action.
"In honor of my bestie Juliana, my assistant Kathy, my grandmother Laura, Mimo and all the people who are fighting and have fought breast cancer," she wrote on Instagram alongside this adorable family selfie of the three with her hairstylist, all rocking pink hair. "Your courage shines and inspires. We decided to show our support by wearing pink proud!"