Simultaneous contrast

Simultaneous contrast refers to the manner in which the colors and brightnesses two different objects affect eachother. The effect is more noticeable when shared between objects of complementary color.

Take for example a red apple under a light source, right next to a green one slightly further from that light source. The red apple will look slightly more green and darker in the areas closer to the green apple, and the green apple will look slightly redder and brighter. .

This is a different concept than contrast, which by itself refers to one object's difference in color and brightness compared to its surroundings or background.