As stated in wikipedia, complementary colors are pairs of colors which, when combined, cancel each other out. This means that when combined, they produce a gray-scale color like white or black. When placed next to each other, they create the strongest contrast for those particular two colors. That's why life vests are often orange (0xFFA500), which is direct opposite of navy blue (0x005AFF). Due to this striking color clash, the term opposite colors is often considered more appropriate than "complementary colors".
Complementary colors can create some striking optical effects. The shadow of an object appears to contain some of the complementary color of the object. For example, the shadow of a red apple will appear to contain a little blue-green. This effect is often copied by painters who want to create more luminous and realistic shadows. Also, if you stare at a square of color for a long period of time (thirty seconds to a minute), and then look at a white paper or wall, you will briefly see an afterimage of the square in its complementary color. Placed side by side as tiny dots, in partitive color mixing, complementary colors appear gray.
Finding complementary color is very simple in RGB model. For any given color, for example, red (#FF0000) you need to find the color, which, after being added to red, creates white (0xFFFFFF). Natually, all you need to do, is substract red from white and get cyan (0xFFFFFF - 0xFF0000 = 0x00FFFF).
To use the calculator, simply add color to the table below. Output table will show original entered color along with its complimentary color.