What could cause red and sometimes purple feet?

Raynauds. It is possible you are having a vaso-spasm. This can cause color changes. I suggest you see your family dr.
Dependent rubor. Is a condition where blood pools in the foot when the legs are in a dependent position. This tends to occur in people with venous insufficiency. Raynaud's syndrome or disease can also cause feet to undergo color changes.

Related Questions

What can cause blueness in hands, purple feet, red spots?

Poor circulation . Autoimmune problems like raynaud's phenomenon. See your doctor for an evaluation. Read more...
Poor blood flow. Vascular diseases ex: raynauds can cause blue hands and feet with exposure to cold or emotional stress. Your condition is best diagnosed by a vascular specialist. Read more...