I suffer from this myself. I did question a doctor on it one time and was told as long as you don't have severe discoloration or a blueish look or it does not cause pain its not a major issue and is very much common for most people to experience it from time to time.
There are many causes and many reasons it can happen to someone. Hyperventilation so you could have a lack of oxygen in the blood or lack of nutrients so it could be poor diet. I was told stress and lack of cardiovascular activity can also cause it.
I think the truth here is it could be so many things and seeing how they are the farthest extremity from the heart they will tend to be colder so if you don't feel its a serious condition i would say just be more active and get the blood flowin any time you feel they need to be warmed up.