3 or more different colours.
Rachel McAdams - Becky Fuller in Morning Glory (Red)
Allie Hamilton in The Notebook (Blonde/Red)
Claire Cleary in Wedding Crashers (Black)
I'm sure there're cases to be made for Portman/Stone/Johansson too.
It's quite a talent, I think i'd just look very, very, ill if I changed my hair to blonde...