Usability studies have shown that white text over dark backgrounds is hard on the eyes. It's just a design thing of form that does not function...
that aside, you're question of text colour white for Google? no impact at all. We do a lot of work with PPC management and PPC fraud detection and thus Google, and no that is not an issue. (
intevix - web information architects)
You would most likely get slammed for writing for the Spider instead of the Participants (we call "users" Participants, since "user" is so cold)...Google does not like content written for the computer. text colour is not an issue.