When you put colors like red and green next to each other, how do they make each other look super bright and stand out?

When colors like red and green are placed next to each other, they make each other look super bright and stand out due to a visual phenomenon called simultaneous contrast. Simultaneous contrast is the effect where the perception of a color is altered or intensified by the presence of an adjacent color. Red and green are complementary colors, which means they are directly opposite each other on the color wheel. Our visual system processes color information according to the opponent process theory. This theory states that our eyes and brain analyze color in terms of opposing pairs: red versus green, blue versus yellow, and black versus white. When you view a red area, the neural pathway sensitive to red is strongly activated, and this simultaneously inhibits the pathway sensitive to green. Conversely, when you view a green area, the green-sensitive pathway is activated, inhibiting the red-sensitive pathway. When red and green are positioned side by side, the strong presence of the red color causes the visual system to enhance the perception of the adjacent green, effectively making the green appear more intensely green than it would in isolation. In the same way, the green color enhances the perceived intensity of the adjacent red. This reciprocal enhancement of their opposing color properties makes both colors appear more saturated, meaning they look purer and more vibrant, and also increases their apparent brightness. The extreme difference in hue at their shared border triggers this powerful opponent response, creating a distinct, almost vibrating visual effect that makes them strongly stand out.