Why does mercury decolourise a gold ring?
The grey colour is an amalgam of mercury and gold.

Mercury forms amalgams with many other metals. Some are used as chemical reagents in laboratory chemistry as they have different properties than the original metals involved. Gold amalgam is much greyer than gold. Silver amalgam has been used in dentistry.

Mercury has been used in the extraction of gold in mining as it can extract gold from low grade ore. But the recovery process involves distilling off the mercury which is dangerous and polluting.

Chemists used to work with mercury a lot and, when they did, they were recommended to remove their wedding rings or other gold jewellery as the mercury would damage the gold. 

Once the amalgam is formed there is no easy way to reverse the process short of resmelting the ring. If the contact with mercury has been brief, you might be able to polish it to remove the amalgam layer, but this won't be easy.