Two colors that are opposite on the color wheel are called complimentary colors or contrasting colors. When they are side by side they create a striking visual effect that draws the eye. An example of this would be orange and blue. Together they create a compelling visual appeal!
Complimentary colors are based on the color wheel, which is divided into primary, secondary, and tertiary colors. The primary colors: red, blue, and yellow are the building blocks of all other colors. By mixing these three colors you can get a multitude of different colors. Secondary colors are created by mixing two primary colors together. The secondary colors are: green, orange, and purple. The tertiary colors are created by mixing primary and secondary colors. These colors sit between the primary and secondary colors on the color wheel.
When you place two complementary colors next to each other, they create a strong visual contrast, as they are on opposite sides of the color wheel. This contrast enhances the individual hue, making each color appear more vibrant and intense.
The high contrast makes bold designs but care must be taken when using such opposite colors so as to not be too visually jarring. You can use a contrasting color as an accent in a composition but if you had equal amounts of both colors this may lead to a chaotic-looking piece of art.
Examples of contrasting colors are: red and green, blue and orange, and purple and yellow.
When you mix contrasting colors together they will cancel each other out and produce beautiful grey tones that can be used in our paintings.
Some of my favorite combinations are:

You can see how lovely the colors are when you let them mix on the paper :)
Here is a painting I did making my own grey by mixing complimentary colors.
I used Phthalo Blue Red Shade with Quinacridone Sienna. I liked adding a bit more touches of blue in some areas of interest. What do you think? Have you tried mixing complementary colors before to make your own greys and black? What is your favorite combination?

In conclusion, complementary colors are visually appealing and can have a very harmonious design. Experimenting with different contrasting colors can help you discover new and interesting neutrals.