When placed next to each other, complementary colors tend to look balanced and are colors opposite to each other on the color wheel. Also, when placed next to each other, they appear to become brighter.
They are also used together in Colour Therapy to give balance. We need the balance of the electric/cool colours and the magnetic/warm colours for our wellbeing and smooth functioning of our bodies.
Complementary colours are opposite each other on the colour wheel:
Violet

Yellow

Result: Violet and Yellow

Indigo

Orange

Result: Indigo and Orange

Blue

Red

Result: Red and Blue

Green

Magenta

Result: Green and Magenta

Yellow

Violet

Result: Yellow and Violet

Orange

Indigo

Result: Orange and Indigo

Red

Blue

Result: Red and Blue

Magenta

Green

Result: Magenta and Green

Magenta is the eighth color and is not part of the visible spectrum, but is a combination of red and violet, thus combining the energies of those two colors. That is to say, that magenta helps us to use our earthly experience and grounding together with intense spiritual awareness. It can help to release past conditioning and help us to move forward.