Successive ContrastStare at the two top circles for 30 seconds, then look at the two identical yellow circles below. Do they look different? Why?
For more information on this phenomenon see The Eye and Successive Contrast.
The same phenomenon - when it happens concurrently is called Simultaneous Contrast. Colors are affected (perceptually changed) by colors around them. Since we rarely experience any color in isolation, Simultaneous Contrast is always a factor in our perception of color.
In this example the two squares in the center are factually the same. As seen in the proof below, the color agent is identical. But the color effect is changed by context. Each background color has the effect of subtracting its own hue from the square in its center. The result is that we perceive a more yellow-orange on the left side and a more red-orange on the right side.
To try it out yourself see Same Same or Different.
For more information see Color Interaction: Simultaneous Contrast.