My feet have been turning purple when sitting/standing. What may cause this?

Raynauds or stasis. There are multiple possibilities for purple discoloration of the feet but the most common are-1. Venous stasis due to pooling of venous blood in the sitting or standing position because of decrease of the calf muscle pump, 2. Raynaud's syndrome which results in color changes of the feet and even hands due to spasm of the small vessels in the toes and hands and, 3. Vasodilation due to medications.