There are four colors of Aeon energy in the game and most cards have a casting cost that require one or more colors as described below.
Blue cards represent knowledge and the harnessing elemental forces such as lightning, storms and ancient machine creatures.
Green cards represents the brutal power and ferocity of nature, including beasts and spirits.
Black cards represent the power over death, and include many undead and demons.
Red cards focus on fire and destruction and include many creatures such as dwarves, orcs and ogres.
A card of multiple colors requires each color Aeon to be cast, assuming it has a casting cost. Currently there are two types of multicolor cards in the game:
Two color cards require both colors to cast. There are also two-color Aeon cards that provide two different aeon colors to your Aeon Field. These do not have a casting cost.
A gold card is all four colors. That means when you cast the card you must have enough aeon energy for all four colors to cast it.
A gray card has no colors. That means when you cast the card you must have enough total aeon energy among all four colors. When cast, one of each color will be used before a second aeon of the same color.