I believe this twitter banner color change story is a beat-up.
I have an old twatter account, even though I am permanently suspended for calling @jack a pedophile.
It too has recently been changed to light grey.
As I am unable to tweet or change any of the settings on my account, I think this points the the change being an automated patch being rolled out by twitter, perhaps being applied to accounts that either haven't ever uploaded an banner image or who have not changed their banner in some time.
It makes little sense to me that a whole bunch of celebs and other people in the world got together to plan a change to their banners without announcing a purpose.
Surely there are other anons who can confirm or deny if the change has affected their acct.