Fighter jets are used by the military in order to defend the nation and to fight in battles. During a battle, the jets will be sent into the air. Different aircraft serve different purposes. Some are used to bomb while others are sent out to scout out the enemy.
Since these military aircrafts are used in battle, they may want to hide from the enemy. However, it is difficult to camouflage a giant airplane especially if the aircraft is in the sky. Therefore, the airplanes and jets are painted a dull gray color in order for it to be difficult to see. They do not want to stand out and be easily spotted by the enemy. That would make them an easy target.