Call it a case of indecision or perhaps a love affair with California surfer culture: Two-tone hair is clearly the do to rock this summer. Drew Barrymore has been spotted trotting around London with peroxide-dipped tips in stark contrast to her dark crown, while British transplant Alexa Chung sported her own bi-level bob at a fragrance party in New York. Rachel Bilson and Leighton Meester have shown off more subtle gradations on the beachy look, and Allison Sarofim’s bleached waves added edge to her Hamptons-appropriate ensemble. Despite the trend’s obvious California references, first-look honors go to Sarah Jessica Parker, who worked the artfully dual-colored style on the set of “Sex and the City 2” in New York last fall.