Cyanosis is a blue coloration of the skin caused by a lack of oxygen in the blood. It can appear anywhere on the skin or mucous membranes. When it appears on the extremities, it usually indicates a local problem, generally a lack of blood circulation. When it appears on the lips or the torso, it generally indicates the patient is not getting enough oxygen. It is easier to see the lighter the skin color of the patient is.
In reality, deoxgenated blood is a much deeper red than oxygenated blood. However, the deep red appears blue when viewed through the skin. The lighter red colour of oxygenated blood looks pink through the skin.
Cyanosis of the veins is a perfectly normal phenomena.
Cynaosis is not very helpful in a differential diagnosis as there are a great number of underlying conditions that can cause it. It can even be caused in some patients by exposure to cold merely because the blood vessels constrict to conserve heat.