The various factions exist in a rock-paper-scissors relationship.
Red units do double (2x) damage to Green units, and normal damage to Red, Purple, and Yellow units but only half (1/2) damage to Blue units.
Green units do double (2x) damage to Blue units and normal damage to Green, Purple, and Yellow units , but only half (1/2) damage to Red units.
Blue units do double (2x) damage to Red units and normal damage to Blue, Purple, and Yellow units, but only half (1/2) damage to Green units.
Yellow and Purple units do double (2x) damage to each other, and normal damage to Red, Green, and Blue units.
You should take these relationships into consideration when you're setting up your squad. While it might be a good idea to have a squad made up of all the different factions in the beginning, as the game progresses, there might be instances where squads with special units or squads made up of only one faction may be a good idea. Read more about these at our Strategies page.