When you are hiking you should always keep in mind that the weather can change very quickly. Of course, if you stay at high altitude it's more risky with this fast weather changes. Even if you are hiking in summertime there can be several degrees below zero and perhaps some snow may fall down.
The normal temperature of a human body is about 37 degrees. When the body reaches a temperature of 35 degrees it's likely that you feel very cold. And further on if your temp decrease more you'll became helpless and that's why it's important to know some basic rules.
You loose between 25-50% of your body heat from your head. Always use a cap when it's cold outside.
Use dry clothes. When you stop change your wet clothes at once. Thanks to the wet clothes your body will be cold very fast. Dress in many layers, then you get an insulating layer of air between every garment.
If you got cold injuries on your skin, white spots, you can heat some water (40 degrees) or just use your own body. Anyway, don't rub it.