Colors As Emblems by E. Goldsmith

Tree of Jesse detail from Reims Cathedral.

       In early art colors were always used symbolically, and until the old traditions were cast aside by later painters, certain colors were always associated with certain subjects and certain personages. In all the old stained glass these rules were scrupulously followed.

• White was the symbol of light, faith, joy, life, and of religious purity, virginity, and innocence. It signified honor and integrity in the judge, humility in the rich man, and chastity in a woman. Christ appears in white after His resurrection and the Virgin wears it in pictures of the assumption.
• Red, the ruby, denoted fire, divine love, the Holy Spirit; royalty, creative power, and heat. Red and white roses are symbols of love and innocence, or love and wisdom. Thus the angel crowns St. Cecilia. Used in the bad sense, red typified blood, hatred, war. Red and white together were the colors of the devil and of purgatory.
• Blue, that of the sapphire, signified heaven, fidelity, constancy, truth. Christ and the Virgin wear the blue mantle typifying heavenly love and heavenly truth. St. John the Evangelist was given the blue tunic and the red mantle.
• Yellow or Gold was the symbol of the sun, of the goodness of God, of marriage and fruitfulness. St. Joseph wears yellow, and St. Peter, in pictures of the apostles, wears a yellow mantle over a blue tunic. Used in the reverse sense, yellow denoted jealousy, deceit, and inconstancy. The traitor Judas wears a dirty, dingy yellow.
• Green, the emerald, the color of spring, symbolized victory and hope - particularly hope of immortality. 
• Violet, the amethyst, signified passion and suffering, or love and truth. It is the color worn by the martyrs, by the Virgin after the crucifixion, by Mary Magdalene as the penitent, and sometimes by Christ after the resurrection.
• Grey is the color of humility, mourning, penance, and accused innocence.
• Black indicated darkness, wickedness, death, and mourning, and was given to Satan. Black and white signified humiliation or mourning, and purity of life, and for this reason was adopted by the Dominicans and Carmelites.