I have to agree with Irene. It does sound like Raynauds Syndrome to me. My fingers turn white but I have heard others say theirs turn blue or purple. Does it happen when your fingers are cold or other times? Ally
Could be heart related, could be something with the circulatory system (vasospasms of some kind), could be secondary to an autoimmune problem like lupus or RA, Raynauds causes some color changes in the hands and feet, sometimes just one or two fingers at a time.
A doctor should be able to pin it down pretty quickly.