Quickest way get a piece of hose pipe connect one end to the hot tap the other to the cold tap. Turn both on the water in the cold tap should be forced back into the tank and the airlock should be forced out with it.
the cold is usually fed direct from the mains not a tank, and i would expect the cold to be at a higher pressure than the hot so the cold is more likely to travel up the hot pipework.
in fact i cant see how you have got an airlock in the cold pipework in the first place.