McMillan said Aniston -- whose "The Rachel" style in the 1990s made her hair iconic -- admires Gisele Bundchen's hair in particular.
"We look at Gisele, the Victoria's Secret models -- I always love their hair," he told People.
"When we talk about a style we want to create together, we can do it in 10 words or less. She'll say something like, 'Burberry with tuxedo jacket,' and I'll be like, 'Parted down the middle in 2009.'"
McMillan joined hair care company Living Proof -- co-owned by Aniston -- as a celebrity stylist this week, People reported Friday.
"Jennifer and I have been very loyal to each other," McMillan said. "It just makes sense -- why wouldn't I stick with her? She trusts the people who work with her."