User blog comment:Xilinoc/Ch. 592 - Marching Out the ZOMBIES 3/@comment-24113610-20140808065406/@comment-3219912-20140808213038

But that doesn't explain why the Shinigami Gigi summoned earlier to fight the Arrancar didn't have dark skin because she turned some of them into zombies then had them kill themselves.

Honestly to me, it's either that the dark skin = length of Zombification since Toshiro and Bambi were both zombified the previous day. Or Kubo just didn't feel like giving the fodder zombies the dark skin and he wanted to emphasize the importance of Toshiro and Bambi.

Since Gigi never explained it, the reason could be anything at this point.