Probably Reynauds, its a circulation issue, any sign of coldish weather my hands go purple and then white, have cut my hands and not felt it till i see the blood, dripping.
Eating ginger or ginger biscuits helps as does eating plenty of meat, youre not a veggie are you, I wear Helly hansen mitts when it gets cold, usually works, also dont wear anything to tight to many socks or gloves as they can stop or reduce the circulation, if you do loose the feeling rub your hands under the COLD tap not hot , or dip yopur hands in a stream or cow trough.
Shaking them gently also helps, as does a warm drink, try tapping your fingers gently on the bars as you cycle along to the music on your ipod.