What's going on?
When you look at a single colour for a long time, the cells in your eyes that are sensitive to this colour get tired, and stop working as much.
When you look away, these cells are quite tired so the brain uses the cells that see the opposite colours.
This is why when you look at something white you see the opposite colour to the colour you were previously looking at!
Image credit: Amy Winehouse, Afterimage, digital drawing by Dimitri Parant is licensed under CC BY-SA 2.0