General Characteristics of Icons
Icons are often called 'windows to heaven' and because time and space are suspended in eternity, icons are two-dimensional (i.e. flat). There is no depth and events which occurred at different times in history are shown together.
The more spiritually important persons are usually larger in size to emphasize their status. There are no shadows since there is no darkness in heaven. The only variation in light appears in the halos that radiate from the holy persons represented (this illustrates that the spiritual light of grace which flows from the heart filled with the grace of the Holy Spirit rather than the floating halos of Western art). A lamp or candle is typically placed before the face to emphasize this glow.
Eyes are written large to show that the holy persons have seen God through faith. Ears are larger and mouths are smaller to emphasize the need to listen to God in silence. Saints face the viewer while others are in profile. Clothing and buildings sometimes reflect Byzantine style rather than first century Jerusalem. Bishop saints are distinguished by their vestments.
The traditional colors for icons are either color pigment in beeswax or egg tempera (the yolk of an egg mixed with an equal amount of water added to the pigment). Mounting on wood is standard. Modern methods often use acrylic (never oil) as well as mass-produced prints. This site contains extensive information on the process involved. Icons also vary slightly by style from country to country (Greek, Cretan, Russian, etc.) but basic rules remain the same.
The colors used in icons also have meaning:
Black: death and in other cases, evil
Blue: infinity and the spiritual world beyond the sky (dark blue is often used for Mary)
Brown: the material, mortal world
Gold: the divine nature of God (brightly radiating outward and dispelling all darkness)
Gray: never used because is a mixture of black and white (vague, unclear, 'lukewarm')
Red: Christ's resurrection, sacrifice and Christian martyrdom (the color of blood)
White: God's divine light, holiness and purity