The "grey floors" thing is born of a trend where house flippers intentionally redo all the interior colors to be as neutral as possible so the houses sell quicker.
Real estate agents will counsel sellers that bright, vibrant colors drive shoppers away. That a neutral (grey) space allows buyers to better visualize their stuff in that space, instead of whatever wacky personal choices the previous owner made.
Zillow and the other real estate sites are crammed with these images: bland, dull, staged neutral spaces that all look the same. "This is what affluence and elegance looks like," we are programmed to believe.
So when a first-time homeowner gets their hands on the controls, their instinct is often to emulate what they've seen for years on HGTV and Zillow: grey floors.