You know those gifs and videos that make you stare at an image for ages and then show you a monochrome one, which your eyes fill in with colour?
Yeah, they're great aren't they?
Here's how they work.
As you stare at an image, light bounces off it and hits your retina - and the rods and cones in the retina interpret colour and tone.
When they do this for a while, the cells in the retinas which are responsible for interpreting different colours become fatigued and stop sending signals to the brain.
What results is a relatively increased activity in the remaining cones - the opposite colour.
So if you switch to a monchrome image, you will see the opposite colour on the black and white image - this is known as after-imaging.
It's better explained by ASAP science as always (who also explain how colour models and screens work too), so watch the full video, below: