Why do my feet turn blue/purple while sitting? I have no pain, when I stand up the color returns.

Vascular related. It is plausible that when you stand and walk the musculature system helps blood flow back thru the venous network but when you sit the blood pools in the feet letting it turn the color of venous blood when it pools. It is worth checking out with a vascular doctor. There may not be much to do for it, but it should be checked out anyway.
Circulation. This is often one of the signs of a circulatory problem. I would recomment a blood flow study performed by a podiatrist or vascular surgeon.