Red has the lowest vibrational energy of all the colors in the rainbow, this means it is closely linked with the physical aspects of life. Because of this it is also linked with the Root Chakra and its alignment to the physical, instinctual side of human existence.
Because it is such a bright, bold color it brings to mind images of passion and violence. It is the color of the delicate red rose with its focus on devotion and desire. This is not the color for soft romantic gentle love. This is deep physical passion.
It is the color of the vital blood that flows through our veins and so is associated with will and war.
In cards it nearly always represents intense passions and physical love and extreme emotions. It can appear as a marker for anger and for an impetus to change.
No matter where the color red might appear it always heralds a vibrant, intense, physical energy.