When i get cold my fingers sometimes become numb and turn white at the tips? why does this happen?

I'm : I'm assuming it's both hands... Without seeing you, my best guess would be some sort of vasospasm of the small blood vessels under your skin. Raynaud's disease / phenomenon is probably the most commonly known vasospastic disease. I'm not sure whether you should start with a neurologist, rheumatologist or more likely, your primary care physician, who'd be able to assess the situation and make a more appropriate referral.