Roughly what you see on the white: faint opposite colors, pink for green and blue-green for red.
Tired eyes
Afterimage
Start the timer and stare at the middle of the picture until it turns white. What do you see?
The colors swap: a pink background with greenish letters.
Why? The cells that sense each color get tired while you stare. On white, the tired ones respond less than the rest, so you see the opposite color for a few seconds.
Picture: Freakmighty, Moxfyre, Public domain · Wikipedia(opens in a new tab)