When complementary colors are used next to each other in a composition, they produce a visual anomaly called simultaneous contrast. this visual effect makes the colors appear to along the boundary where the two colors meet?

Simultaneous contrast describes the manner how two different object affect each other. The effect is more noticeable when shared between objects of complementary color.
The real colors are not altered; only our perception of them changes.
This visual effect makes the colors appear to be lighter or darker than are in isolation, along the boundary where the two colors meet. The colors around the color determine if the color we are interested in, will look darker or lighter.