Simultaneous contrast, is when the effect of a color is either enhanced or diminished by the surrounding colors in a piece of art.  The effect is a little bit more visible when the colors are complementing each other. In the piece on the left, the lighter orange background, makes the blue squares seem darker, whereas, the darker background makes the blue seem darker, although it is the same shade.

One challenge I had creating this piece was ensuring the effect of the simultaneous concept, was visible by choosing the correct colors and getting the right values as perception plays a key role in simultaneous contrasting art.