Why do complementary colors stand out?
Short answer
Complementary colors are roughly opposite one another on a color wheel. When colors with different properties sit next to each other, their contrast can make each color appear more noticeable.
How it works
Pairs such as red and green or blue and orange have a large difference in hue. The eyes and brain compare neighboring colors, so the same color can look different depending on the colors around it.
Noticeability is not determined by hue alone. Brightness, size, type weight, and the background also affect whether something is easy to see or read.
Key point
Complementary colors are useful for strong contrast, but too much contrast can feel harsh. Designers also adjust area and brightness to fit the purpose.