Gray is considered neutral and is complementary with any other color. Gray functions in a way that is similar to black and white when it comes to complementing other colors.
Technically speaking, complementary colors are those positioned opposite from one another on the color wheel. As is the case with black and white, gray is not a true color and does not appear on a standard color wheel. In addition to complementing any color on the color wheel, gray also readily complements black and white. If gray is added to a color on the color wheel, a different tone of that color is created.