The tetradic colour scheme makes use of a combination of four colours, which sit opposite each other on the colour wheel in two complementary pairs.
This is a rich color scheme which offers plenty of possibilities for variation, but it can be hard to balance. You need to be aware of the warm/cool balance as well as avoiding the pure chroma and work with a similar tint or shade of all four colours instead.
One way to make this many colours in a combination work is to use white space and a neutral background, as Suz has done in this tribute page to her sons favourite bands. She has mixed the unlikely combination of yellow-orange and green, with their opposite purples and blues. Roughly equal quantities of each colour makes for a dynamic combination, which is what Suz was going for with this layout.
Kathie’s used a tetradic colour scheme of blue, green-yellow, orange and red-violet for her layout. A tetradic colour combination can be quite a rich colour scheme, so Kathie’s style of using a neutral backdrop and loads of white space works well for this particular colour combination.
The other trick to using a tetradic combination is to focus on a couple of the colours and use the others as accents. You can see that Kathie has focused on the blue and green in the colour scheme, introducing splashes of the red and orange with some of the embellishments.