The principles of colour theory are used to create harmonious colour schemes. Colour Theory is based on Sir Isaac Newton’s original colour wheel and takes into account the psychological effect of colours and their combinations.
The classic colour combinations which form the “rules of thumb” of colour theory are:

Monochromatic
The monochromatic colour scheme is a simple, out-of-the-box match. Instead of using different colours, it simply uses variations of a single colour. A monochromatic colour scheme is very easy on the eyes, but keep an eye on your contrasts.

Analogous
Analogous colour schemes combine colours close to each other on the colour wheel, eg. red & orange; green & blue. Simply a richer version of the monochromatic colour scheme, an analogous combination is also easy on the eyes, but lacks contrast.

Complementary
A complementary colour scheme is a striking combination which uses two colours that sit opposite each other on the colour wheel, eg. red & green; blue & yellow. It is a challenging combination to achieve harmonious aesthetics with, but creates a strong, high-contrast effect.
Split Complementary
The split complementary colour scheme adds variety to and reduces the harshness of the complementary colour scheme by allowing additional colours, close to one of the complementary colours on the wheel.

Triadic
The triadic colour scheme uses three colours, equally apart from each other on the colour wheel. Each colour balances the other two, creating a harmonious palette for your website.
Tetradic
The tetradic colour scheme can also be referred to as the “double complementary”, as it uses two complementary colour sets.
Of course, as with everything, colour theory is simply a set of textbook rules used as a guide. Any combination of colours can look great and achieve your desired effect when properly used.
External Links:
Color Wheel Pro: Color Theory Basics
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