Successive Contrast

Choose either colored circle and stare at the + for 30 seconds. Then scroll down and look at the + on the corresponding orange circle below.

The afterimages of the red and cyan circles float above the orange circles. Those afterimages are opposite to the stimulus color, so red becomes cyan and cyan becomes red. 

The floating afterimages mix with the stimulus color apparent color of the orange circles, making the two orange circles appear different from each other until the eye readapts.

This effect is known as successive contrast.