Viewing two colors at the same time influences both of their appearances. The following is an example of simultaneous contrast.
It is easy for me to state that the gray bar is exactly the same shade of gray throughout it's range. Most of us however will swear that the bar varies in color as it progresses across the page. If you have access to a color identifier program you will find that the bar is color 7e7e7e at every point within it's border. Otherwise put the RGB value is R126 G126 B126.
Simultaneous Contrast Illusion.
The gray bar is the same shade throughout.