Complementary colors

Complementary colors are pairs of colors which, when combined, cancel each other out. This means that when combined, they produce a grey-scale color like white or black. When placed next to each other, they create the strongest contrast for those particular two colors. Due to this striking color clash, the term opposite colors is often considered more appropriate than "complementary colors".

Which pairs of colors are considered complementary depends on the color theory one uses:

• Modern color theory uses either the subtractive color model, or the additive color model, and in these, the complementary/opposite pairs are red & cyan, green & magenta, and blue & yellow.
• In the traditional RYB color model, the complementary/opposite color pairs are red & green, yellow & violet, and blue & orange, though these pairings fail the modern definition of complementary colors, as they produce a brown color when combined.
• Opponent process theory suggests that the most contrasting color pairs are red & green, and blue & yellow.

https://en.wikipedia.org/wiki/Complementary_colors