Judging by her list of A-list clients, the reigning queen of the celebrity haircolor scene is Tracey Cunningham. Her chair at Meche Salon in Beverly Hills is often occupied by a high-profile star like Jennifer Lopez, Cameron Diaz, Jessica Biel or Drew Barrymore. She's also likely to pack up her brushes and bowls and head to the airport when she receives a haircolor S.O.S. from Shakira in Barcelona or Sienna Miller in London. Needless to say, Tracey not only knows the trends, she sets them. This summer, she reveals, the word to keep in…