Tetrad:

Here is the next term

8.         Tetrad: Four equilateral hues from the color circle (i.e. R, O, G, B)

R.D.V. Ihadulca from Evil Zone for the Villains Contest at FGE. Digital image. 

FGE / Game Art HQ Contest Entries. Game-Art.HQ, 2011-2012. Web. 11 Dec. 2012

I use this pic because of the four color group green, red, purple, and orange.