The Father is represented in the figure on the left, the robes He wears are primarily gold, representing His Kingship and royalty.
Jesus, the Son, both fully Divine and fully human wears brown, the color of the earth. The Spirit on the right, wears both blue and green to symbolize the Spirit's moving through both sky and water.
Notice the rectangle at the table, between the Father and the Spirit? Historians believe that at one time there was a mirror there, inviting a fourth person to the table. Inviting us to to join in the divine circle of love that is God.
Here's more information about this icon.
