How will the altimeter read in air colder than ISA?
Air expands as the temperature increases and it compresses when the temperature decreases. 



image source: aviationweather.ws

When it is colder than ISA, the air column is compressed and therefore you are flying lower than the altimeter is indicating. In other words: the altimeter will over read in cold air. 

So to fly safely over an obstacle at a cold day, you have to add extra margin to the obstacle's height, because your altimeter will show a higher altitude than your true altitude.