In human color psychology, red is associated with heat, energy, and blood, and emotions that "stir the blood", including anger, passion, love and pain.
The color red is associated with lust, passion, love, and beauty as well. The association with love and beauty is possibly related to the use of red roses as a love symbol. Red is frequently used as a symbol of guilt, sin and anger, often as connected with blood or sex.
Red is also used as a symbol of courage and sacrifice. Red may represent fire and so may symbolize the presence of God.
Red is one of the most common colors used on national flags throughout the world. The use of red has some similar connotations from country to country: the blood, sacrifice, and courage of those who defended their country; the sun and the hope and warmth it bring; and the sacrifice of Christ's blood.