Afterimages
In colour vision, if we experience prolonged exposure to a colour (or set of colours in our visual field), when that colour stimulus is removed, we perceive the same image but in a very different colour, which is one conception of complementary colours. This perceptual image is generated by our eyes and brain. (Note: this is more precisely termed a negative afterimage and is also called successive contrast.)
Simple definition: If we stare at a strong colour for 15 seconds or more, then look away to a neutral surface, we then see a very different colour.
Example: Stare at the X inside the green coloured circle and count to 30, then move your eyes to the X beside the circle. What do you see? Also try looking at a white piece of paper instead of the white coloured screen.
To experience an afterimage, stare at the X in the circle on the left for 30 seconds, then look at the X on the white background on the right.
