Call it a case of indecision or perhaps a love affair with California surfer culture: Two-tone hair is clearly the do to rock before summer ends. Drew Barrymore has been spotted trotting around London with peroxide-dipped tips in stark contrast to her dark crown. 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. Overgrown roots have never looked so good.