Colour schemes are a major aspect of web design.

Colour wheels, such as the one above, are often used to help web designers choose a colour scheme for their site.
There are two main types of colour scheme:

1. Complementary
2. Triadic

Complementary colour schemes involve colours that are on opposite sides of the colour wheel. These naturally go well with eachother, hence the name "complementary".

Triadic colour schemes use colours that are adjacent in the colour wheel, so they blend well together. You typically use three colours, a dominant one the two on either side in relation to the colour wheel.